Add support for protobuf TxGenerator and SIGN_MODE_DIRECT

[aaronc]

ref: #6213

Summary

This PR adds support for a protobuf client.TxGenerator with signing via SIGN_MODE_DIRECT.

Details

• Protobuf Tx additions/changes:
  • Add implementations of client.TxGenerator and client.TxBuilder for the new protobuf Tx in the new x/auth/tx package (the old StdTx will move to x/auth/tx/legacy in a future PR)
  • Add a SignModeHandler implementation for SIGN_MODE_DIRECT
  • Adds the TxRaw proto type and updates SignDoc as well as doc comments to reflect the latest state of ADR 020, updates SignerInfo to use PublicKey rather than Any.
• Refactoring of existing code:
  • Move sdk.TestMsg to codec/testdata adding protobuf support
  • Move the x/auth/ante.SigVerifiableTx to x/auth/signing.SigVerifiableTx so that it can be used from other packages
  • Add the SigFeeMemoTx interface to x/auth/signing which wraps sdk.Tx, FeeTx, TxWithMemo and SigVerifiableTx in a single interface and use this in client.TxBuilder. (I'm not sure this is well named... but functionally it makes sense because this is what TxBuilder actually supports and is standard throughout the SDK.)
  • Rename CompactBitArray.Size() to Count() and re-enable gogoproto.sizer - this was causing a nasty bug and the proto Size method shouldn't have been overridden.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[codecov]

Codecov Report

Merging #6385 into master will increase coverage by 0.26%.
The diff coverage is 72.62%.

@@            Coverage Diff             @@
##           master    #6385      +/-   ##
==========================================
+ Coverage   57.04%   57.30%   +0.26%     
==========================================
  Files         481      491      +10     
  Lines       28939    29470     +531     
==========================================
+ Hits        16507    16889     +382     
- Misses      11258    11372     +114     
- Partials     1174     1209      +35     

[aaronc]

@fedekunze addressed all your review comments. The test coverage should be better now.

@alexanderbez the only pending thing I see is figuring out this naming thing. What do you think?

Would be good to get this merged soon to keep #6213 moving.

[fedekunze]

LGTM