Add SignatureV2 infrastructure

[aaronc]

This is one of several bite-size PR's that are being pulled out of #6216 to implement #6213.

Summary

This PR adds the basic application logic types for dealing with multiple signing modes as well as multi-signatures in a backwards compatible and hopefully convenient way. The top-level new types here are SignatureV2 to be used for both signing and verifying and the multisig.PubKey interface.

Details

• SignMode was moved into a separate package (types/tx/signing) because of cyclic dependency issues
• SignatureV2 and Single|MultiSignatureData types were added
• multisig.PubKey interface was added which uses MultiSignatureData for verification
• multisig.Multisignature was renamed to AminoMultisignature and convenience functions for converting between amino StdSignature/AminoMultisignature and new SignatureV2/SignatureData types were added
• multisig.AddSignature* functions now use MultiSignatureData instead of AminoMultisignature

---

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 #6373 into master will increase coverage by 0.01%.
The diff coverage is 72.29%.

@@            Coverage Diff             @@
##           master    #6373      +/-   ##
==========================================
+ Coverage   55.66%   55.67%   +0.01%     
==========================================
  Files         465      465              
  Lines       27674    27782     +108     
==========================================
+ Hits        15404    15469      +65     
- Misses      11169    11206      +37     
- Partials     1101     1107       +6     

[sahith-narahari]

@fedekunze I addressed the comments, please take a look

[fedekunze]

ACK, minor comments