fix(x/tx): fix amino json drift from legacy spec (backport #21825)

[mergify]

Description

Related to: #21782

Recent changes had caused all amino JSON legacy equivalence rapid tests to early out before running. As a consequence changes to the x/tx Amino JSON encoder were not properly tested, and several issues were not caught.

They are:

1. Field ordering was incorrect in some cases (including oneofs)
2. Decimals were rendering incorrectly

The tests and specs for amino JSON encoding have been tightened up. The tests now closely mimic the same code paths followed in the updated TxDecoder in x/tx.

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Summary by CodeRabbit

• New Features

  • Introduced a new framework for testing transaction signing functionalities, enhancing the testing capabilities for compatibility and correctness.

• Bug Fixes

  • Improved error handling in encoding functions to manage invalid inputs and ensure consistent output formats.
  • Updated decimal representation in tests for better precision.

• Refactor

  • Streamlined test setup and logic for message generation and validation, enhancing code readability and maintainability.
  • Modularized custom signer logic for better maintainability.
  • Restructured encoding functions to improve flexibility and error handling.

• Chores

  • Removed unused imports and redundant error handling to contribute to a cleaner codebase.
  • Added a new entry in the changelog for the x/tx module regarding recent fixes.

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This is an automatic backport of pull request #21825 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2d40cc1 has failed:

On branch mergify/bp/release/v0.50.x/pr-21825
Your branch is up to date with 'origin/release/v0.50.x'.

You are currently cherry-picking commit 2d40cc1ab.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   tests/integration/rapidgen/rapidgen.go
	modified:   tests/integration/tx/context_test.go
	new file:   tests/integration/tx/internal/util.go
	modified:   x/tx/signing/aminojson/encoder.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tests/integration/tx/aminojson/aminojson_test.go
	both modified:   x/auth/migrations/legacytx/stdtx_test.go
	both modified:   x/tx/CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[julienrbrt]

I had to revert the tests here. bumped x/tx to a pseudo version from this branch before tagging after this PR.

@kocubinski, does it make sense to re-add them for 0.50? If so, could you please to it, it seems like it differed quite a bit from 0.52/main.