refactor: WriteAcknowledgement API (cosmos#882)

* refactor: WriteAcknowledgement takes exported.Acknowledgement instead of bytes

* fix: adding check for empty byte string

* chore: update changelog

* fixing test case + adding migration docs

* testing: Adding MockEmptyAcknowledgement to testing library

* docs: fix version

* test: add check for ack is nil

CHANGELOG.md

@@ -56,7 +56,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (transfer) [\#517](https://github.com/cosmos/ibc-go/pull/517) Separates the ICS 26 callback functions from `AppModule` into a new type `IBCModule` for ICS 20 transfer.
 * (modules/core/02-client) [\#536](https://github.com/cosmos/ibc-go/pull/536) `GetSelfConsensusState` return type changed from bool to error.
 * (channel) [\#644](https://github.com/cosmos/ibc-go/pull/644) Removes `CounterpartyHops` function from the ChannelKeeper.
-* (testing) [\#776](https://github.com/cosmos/ibc-go/pull/776) Adding helper fn to generate capability name for testing callbacks 
+* (testing) [\#776](https://github.com/cosmos/ibc-go/pull/776) Adding helper fn to generate capability name for testing callbacks
 * (testing) [\#892](https://github.com/cosmos/ibc-go/pull/892) IBC Mock modules store the scoped keeper and portID within the IBCMockApp. They also maintain reference to the AppModule to update the AppModule's list of IBC applications it references. Allows for the mock module to be reused as a base application in middleware stacks.
 * (channel) [\#882](https://github.com/cosmos/ibc-go/pull/882) The `WriteAcknowledgement` API now takes `exported.Acknowledgement` instead of a byte array
 * (modules/core/ante) [\#950](https://github.com/cosmos/ibc-go/pull/950) Replaces the channel keeper with the IBC keeper in the IBC `AnteDecorator` in order to execute the entire message and be able to reject redundant messages that are in the same block as the non-redundant messages.
@@ -76,13 +76,13 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (channel) [\#692](https://github.com/cosmos/ibc-go/pull/692) Minimize channel logging by only emitting the packet sequence, source port/channel, destination port/channel upon packet receives, acknowledgements and timeouts.
 * [\#383](https://github.com/cosmos/ibc-go/pull/383) Adds helper functions for merging and splitting middleware versions from the underlying app version.
 * (modules/core/05-port) [\#288](https://github.com/cosmos/ibc-go/issues/288) Making the 05-port keeper function IsBound public. The IsBound function checks if the provided portID is already binded to a module.
-* (channel) [\#644](https://github.com/cosmos/ibc-go/pull/644) Adds `GetChannelConnection` to the ChannelKeeper. This function returns the connectionID and connection state associated with a channel. 
-* (channel) [\647](https://github.com/cosmos/ibc-go/pull/647) Reorganizes channel handshake handling to set channel state after IBC application callbacks. 
-* (client) [\#724](https://github.com/cosmos/ibc-go/pull/724) `IsRevisionFormat` and `IsClientIDFormat` have been updated to disallow newlines before the dash used to separate the chainID and revision number, and the client type and client sequence. 
+* (channel) [\#644](https://github.com/cosmos/ibc-go/pull/644) Adds `GetChannelConnection` to the ChannelKeeper. This function returns the connectionID and connection state associated with a channel.
+* (channel) [\647](https://github.com/cosmos/ibc-go/pull/647) Reorganizes channel handshake handling to set channel state after IBC application callbacks.
+* (client) [\#724](https://github.com/cosmos/ibc-go/pull/724) `IsRevisionFormat` and `IsClientIDFormat` have been updated to disallow newlines before the dash used to separate the chainID and revision number, and the client type and client sequence.
 
 ### Features
 
-* [\#432](https://github.com/cosmos/ibc-go/pull/432) Introduce `MockIBCApp` struct to the mock module. Allows the mock module to be reused to perform custom logic on each IBC App interface function. This might be useful when testing out IBC applications written as middleware. 
+* [\#432](https://github.com/cosmos/ibc-go/pull/432) Introduce `MockIBCApp` struct to the mock module. Allows the mock module to be reused to perform custom logic on each IBC App interface function. This might be useful when testing out IBC applications written as middleware.
 * [\#380](https://github.com/cosmos/ibc-go/pull/380) Adding the Interchain Accounts module v1
 * [\#679](https://github.com/cosmos/ibc-go/pull/679) New CLI command `query ibc-transfer denom-hash <denom trace>` to get the denom hash for a denom trace; this might be useful for debug
 
@@ -161,7 +161,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 ### API Breaking
 
 * (core) [\#227](https://github.com/cosmos/ibc-go/pull/227) Remove sdk.Result from application callbacks
-* (transfer) [\#350](https://github.com/cosmos/ibc-go/pull/350) Change FungibleTokenPacketData to use a string for the Amount field. This enables token transfers with amounts previously restricted by uint64. Up to the maximum uint256 value is supported. 
+* (transfer) [\#350](https://github.com/cosmos/ibc-go/pull/350) Change FungibleTokenPacketData to use a string for the Amount field. This enables token transfers with amounts previously restricted by uint64. Up to the maximum uint256 value is supported.
 
 ### Features
 
@@ -293,7 +293,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 ## [v1.1.2](https://github.com/cosmos/ibc-go/releases/tag/v1.1.2) - 2021-10-15
 
 * [\#485](https://github.com/cosmos/ibc-go/pull/485) Bump SDK version to v0.44.2
- 
+
 ## [v1.1.1](https://github.com/cosmos/ibc-go/releases/tag/v1.1.1) - 2021-10-04
 
 ### Dependencies
@@ -324,8 +324,8 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (core) [\#200](https://github.com/cosmos/ibc-go/pull/200) Fixes incorrect export of IBC identifier sequences. Previously, the next identifier sequence for clients/connections/channels was not set during genesis export. This resulted in the next identifiers being generated on the new chain to reuse old identifiers (the sequences began again from 0).
 * (02-client) [\#192](https://github.com/cosmos/ibc-go/pull/192) Fix IBC `query ibc client header` cli command. Support historical queries for query header/node-state commands.
 * (modules/light-clients/06-solomachine) [\#153](https://github.com/cosmos/ibc-go/pull/153) Fix solo machine proof height sequence mismatch bug.
-* (modules/light-clients/06-solomachine) [\#122](https://github.com/cosmos/ibc-go/pull/122) Fix solo machine merkle prefix casting bug. 
-* (modules/light-clients/06-solomachine) [\#120](https://github.com/cosmos/ibc-go/pull/120) Fix solo machine handshake verification bug. 
+* (modules/light-clients/06-solomachine) [\#122](https://github.com/cosmos/ibc-go/pull/122) Fix solo machine merkle prefix casting bug.
+* (modules/light-clients/06-solomachine) [\#120](https://github.com/cosmos/ibc-go/pull/120) Fix solo machine handshake verification bug.
 * (modules/light-clients/06-solomachine) [\#153](https://github.com/cosmos/ibc-go/pull/153) fix solo machine connection handshake failure at `ConnectionOpenAck`.
 
 ### API Breaking
@@ -334,19 +334,19 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (modules) [\#206](https://github.com/cosmos/ibc-go/pull/206) Expose `relayer sdk.AccAddress` on `OnRecvPacket`, `OnAcknowledgementPacket`, `OnTimeoutPacket` module callbacks to enable incentivization.
 * (02-client) [\#181](https://github.com/cosmos/ibc-go/pull/181) Remove 'InitialHeight' from UpdateClient Proposal. Only copy over latest consensus state from substitute client.
 * (06-solomachine) [\#169](https://github.com/cosmos/ibc-go/pull/169) Change FrozenSequence to boolean in solomachine ClientState. The solo machine proto package has been bumped from `v1` to `v2`.
-* (module/core/02-client) [\#165](https://github.com/cosmos/ibc-go/pull/165) Remove GetFrozenHeight from the ClientState interface. 
+* (module/core/02-client) [\#165](https://github.com/cosmos/ibc-go/pull/165) Remove GetFrozenHeight from the ClientState interface.
 * (modules) [\#166](https://github.com/cosmos/ibc-go/pull/166) Remove GetHeight from the misbehaviour interface. The `consensus_height` attribute has been removed from Misbehaviour events.
-* (modules) [\#162](https://github.com/cosmos/ibc-go/pull/162) Remove deprecated Handler types in core IBC and the ICS 20 transfer module. 
+* (modules) [\#162](https://github.com/cosmos/ibc-go/pull/162) Remove deprecated Handler types in core IBC and the ICS 20 transfer module.
 * (modules/core) [\#161](https://github.com/cosmos/ibc-go/pull/161) Remove Type(), Route(), GetSignBytes() from 02-client, 03-connection, and 04-channel messages.
 * (modules) [\#140](https://github.com/cosmos/ibc-go/pull/140) IsFrozen() client state interface changed to Status(). gRPC `ClientStatus` route added.
 * (modules/core) [\#109](https://github.com/cosmos/ibc-go/pull/109) Remove connection and channel handshake CLI commands.
-* (modules) [\#107](https://github.com/cosmos/ibc-go/pull/107) Modify OnRecvPacket callback to return an acknowledgement which indicates if it is successful or not. Callback state changes are discarded for unsuccessful acknowledgements only. 
+* (modules) [\#107](https://github.com/cosmos/ibc-go/pull/107) Modify OnRecvPacket callback to return an acknowledgement which indicates if it is successful or not. Callback state changes are discarded for unsuccessful acknowledgements only.
 * (modules) [\#108](https://github.com/cosmos/ibc-go/pull/108) All message constructors take the signer as a string to prevent upstream bugs. The `String()` function for an SDK Acc Address relies on external context.
 * (transfer) [\#275](https://github.com/cosmos/ibc-go/pull/275) Remove 'ChanCloseInit' function from transfer keeper. ICS20 does not close channels.
 
 ### State Machine Breaking
 
-* (modules/light-clients/07-tendermint) [\#99](https://github.com/cosmos/ibc-go/pull/99) Enforce maximum chain-id length for tendermint client. 
+* (modules/light-clients/07-tendermint) [\#99](https://github.com/cosmos/ibc-go/pull/99) Enforce maximum chain-id length for tendermint client.
 * (modules/light-clients/07-tendermint) [\#141](https://github.com/cosmos/ibc-go/pull/141) Allow a new form of misbehaviour that proves counterparty chain breaks time monotonicity, automatically enforce monotonicity in UpdateClient and freeze client if monotonicity is broken.
 * (modules/light-clients/07-tendermint) [\#141](https://github.com/cosmos/ibc-go/pull/141) Freeze the client if there's a conflicting header submitted for an existing consensus state.
 * (modules/core/02-client) [\#8405](https://github.com/cosmos/cosmos-sdk/pull/8405) Refactor IBC client update governance proposals to use a substitute client to update a frozen or expired client.

docs/migrations/v3-to-v4.md

@@ -0,0 +1,26 @@
+# Migrating from ibc-go v2 to v3
+
+This document is intended to highlight significant changes which may require more information than presented in the CHANGELOG.
+Any changes that must be done by a user of ibc-go should be documented here.
+
+There are four sections based on the four potential user groups of this document:
+- Chains
+- IBC Apps
+- Relayers
+- IBC Light Clients
+
+**Note:** ibc-go supports golang semantic versioning and therefore all imports must be updated to bump the version number on major releases.
+```go
+github.com/cosmos/ibc-go/v3 -> github.com/cosmos/ibc-go/v4
+```
+
+No genesis or in-place migrations required when upgrading from v1 or v2 of ibc-go.
+
+## Chains
+
+### IS04 - Channel 
+
+The `WriteAcknowledgement` API now takes the `exported.Acknowledgement` type instead of passing in the acknowledgement byte array directly. 
+This is an API breaking change and as such IBC application developers will have to update any calls to `WriteAcknowledgement`. 
+
+