Consider reverting sdk.Msg#GetSigners(): string back to sdk.AccAddress

[amaury1093]

Summary

Revert this breaking change introduced in v0.44

type sdk.Msg interface {
  GetSigners() string
}

back to what was in <=0.43

type sdk.Msg interface {
  GetSigners() []sdk.AccAddress
}

introduced in #9239

Problem Definition

Part of the x/auth refactor WG (#7091), we were discussing how to work on it with minimal breaking changes on 0.44. But in 0.44 (master) we introduced #9239, which is a pretty huge API-breaking change.

Proposal

Revert #9239 on master for 0.44.

Then, in the future (0.45+), create a new issue part of the WG to define a new Msg interface

Preliminary ideas

1. Deprecate sdk.Msg
2. In the spirit of moving to core, create core/tx package with:

type Msg interface {
  ValidateBasic() error
  GetSignerAddresses() string
}

and slowly replace sdk.Msg with tx.Msg. More details to be ironed out.

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[aaronc]

This is probably a good idea... unfortunately.

[amaury1093]

I'm triaging this into the ready column for subteams to take on it.