fix(client/v2): *big.Int unmarshal (backport #21853)

[mergify]

Description

Closes: #20935

I am mostly curious about what has changed between 0.50 to 0.52/main x/tx, which makes us need those extra encoders. For instance marshalling was working fine on 0.50, but regressed in 0.52. Fixed by #22161

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title, you can find examples of the prefixes below:

• confirmed ! in the type prefix if API or client breaking change

• targeted the correct branch (see PR Targeting)

• provided a link to the relevant issue or specification

• reviewed "Files changed" and left comments if necessary

• included the necessary unit and integration tests

• added a changelog entry to CHANGELOG.md

• updated the relevant documentation or specification, including comments for documenting Go code

• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

Please see Pull Request Reviewer section in the contributing guide for more information on how to review a pull request.

I have...

• confirmed the correct type prefix in the PR title
• confirmed all author checklist items have been addressed
• reviewed state machine logic, API design and naming, documentation is accurate, tests and test coverage

Summary by CodeRabbit

Summary by CodeRabbit

• New Features

  • Added support for off-chain signing and verification of files.
  • Introduced support for governance proposals.
  • Launched a transaction factory for enhanced transaction management.
  • Extended keyring interface with new types for improved functionality.
  • Enhanced command building with new scalar type handling for decimal values.

• Bug Fixes

  • Resolved issues with unmarshalling and marshalling *big.Int in transactions.
  • Improved error messages for better clarity during transaction handling.

• Documentation

  • Updated changelog to reflect new features and bug fixes.

---

This is an automatic backport of pull request #21853 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 43c41be has failed:

On branch mergify/bp/release/v0.52.x/pr-21853
Your branch is up to date with 'origin/release/v0.52.x'.

You are currently cherry-picking commit 43c41be13.
  (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:   client/v2/autocli/flag/builder.go
	new file:   client/v2/autocli/flag/legacy_dec.go
	modified:   x/auth/tx/builder.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   client/v2/CHANGELOG.md
	deleted by us:   x/tx/decode/decode.go

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