Add new interim governance commands: {create, answer, verify}-poll

  The Cardano Foundation proposes a mechanism for polling Cardano stake
  pool operators on specific topics. Polls are done on-chain through
  transaction metadata and authenticated through stake pool credentials
  (either VRF public key similar to what's described in
  [CIP-0022](https://cips.cardano.org/cips/cip22) or Ed25519 cold key).

  The goal is to gather opinions on governance matters such as protocol
  parameters updates. This standard is meant to be an inclusive interim
  solution while the work on a larger governance framework such as
  [CIP-1694](cardano-foundation/CIPs#380)
  continues.

  See [proposed CIP](cardano-foundation/CIPs#496) for details.

  ---

  This commits adds three new commands:

  - create-poll:
      For the current governing entities, as a means to create new polls.

  - answer-poll:
      For participants who want to answer a given poll.

  - verify-poll:
      For anyone who seek to verify a poll entry (e.g. explorers)

  The commands are built to fit and play nicely within the cardano-cli.
  The poll and answers structures are based on transaction metadata and
  require to be embedded in an actual transaction. The added commands
  however only works from metadata and raw "GovernancePoll" envelopes.

cardano-api/cardano-api.cabal

@@ -66,6 +66,7 @@ library
                         Cardano.Api.Error
                         Cardano.Api.Fees
                         Cardano.Api.GenesisParameters
+                        Cardano.Api.Governance.Poll
                         Cardano.Api.Hash
                         Cardano.Api.HasTypeProxy
                         Cardano.Api.IPC

cardano-api/src/Cardano/Api.hs

@@ -319,10 +319,12 @@ module Cardano.Api (
     -- * Transaction metadata
     -- | Embedding additional structured data within transactions.
     TxMetadata(..),
+    AsTxMetadata(..),
 
     -- ** Constructing metadata
     TxMetadataValue(..),
     makeTransactionMetadata,
+    mergeTransactionMetadata,
 
     -- ** Validating metadata
     validateTxMetadata,