REST tx endpoint backwards compatibility

[sahith-narahari]

Description

Migrates x/auth encode, decode and broadcast REST endpoints, as well as REST tx endpoints in general.

This does conversion to the native tx format (protobuf or amino) behind the scenes allowing existing clients to still submit amino JSON StdTx transactions to protobuf blockchains.

ref: #6213

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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
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #6801 into master will decrease coverage by 0.16%.
The diff coverage is 18.33%.

@@            Coverage Diff             @@
##           master    #6801      +/-   ##
==========================================
- Coverage   61.56%   61.40%   -0.17%     
==========================================
  Files         511      517       +6     
  Lines       31879    32050     +171     
==========================================
+ Hits        19626    19679      +53     
- Misses      10701    10802     +101     
- Partials     1552     1569      +17     

I picked up the PR

[aaronc]

Sorry about the review request chatter. There's a bug on github mobile

[amaury1093]

reviewed:

• client/tx/legacy.go and tests
• x/auth/client/rest/{decode,encode} and tests

they lgtm, though I'm not super familiar with the code either.

[aaronc]

@fedekunze I have did what I could to increase test coverage including:

• covering both amino and proto cases with unit tests in the same build where possible
• adding an integration test for broadcast (previously encode, decode and broadcast were all missing integration tests)

There are likely still some cases that are hard to trigger with a test (such as passing in a Tx which doesn't implement SigFeeMemoTx because there isn't one), but my sense is that most things are now better covered.

Let me know what you think.