feat(x/tx): extract signers using cosmos.msg.v1.signer

[aaronc]

Description

Ref: #11275

This adds a method for extracting the list of signers from a message based on the cosmos.msg.v1.signer protobuf option. It will allow us to make the GetSigners() []sdk.AccAddress method optional which is an important part of the bech32 global removal. This method will also be used by autocli/hubl in the short term for transaction signing support.

This code is more complicated than I had hoped because cosmos.msg.v1.signer supports both nested and repeated signer fields so there are more cases to handle. The functions for extracting the signers are cached the first time they are created to avoid constantly iterating through the logic of the 6 different ways nested and repeated fields can be combined.

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[tac0turtle]

it looks good me. how would this get plugged into the app?

[aaronc]

it looks good me. how would this get plugged into the app?

In an app it would need to be used from the ante handlers to get the signers. There is some other work around making GetSigners optional that I intend to address in a follow up PR

[amaury1093]

LGTM, few nits

[aaronc]

Tagging x/tx/v0.2.1 with this