Add basic sign mode infrastructure

[aaronc]

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

This PR adds:

• SignModeHandler and SigningData types
• LegacyAminoJSONHandler as the first concrete SignModeHandler implementation
• SignModeHandlerMap which aggregates multiple handlers together

This PR does node add support for SIGN_MODE_DIRECT which will be added in a future PR.

---

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 #6371 into master will increase coverage by 0.03%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master    #6371      +/-   ##
==========================================
+ Coverage   55.57%   55.61%   +0.03%     
==========================================
  Files         452      454       +2     
  Lines       27322    27360      +38     
==========================================
+ Hits        15185    15215      +30     
- Misses      11046    11050       +4     
- Partials     1091     1095       +4     

[alexanderbez]

ACK