fix: possible overflow in BuildUnsignedTx (backport #1303)

[mergify]

Description

• (audit issue) fix possible overflow in BuildUnsignedTx
  • referenced implementation(fix(client/tx): avoid integer uint64->int64 overflow by big.Int conversion cosmos/cosmos-sdk#18622)

Motivation and context

How has this been tested?

Screenshots (if appropriate):

Checklist:

• I followed the contributing guidelines and code of conduct.
• I have added a relevant changelog to CHANGELOG.md
• I have added tests to cover my changes.
• I have updated the documentation accordingly.
• I have updated API documentation client/docs/swagger-ui/swagger.yaml

---

This is an automatic backport of pull request #1303 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of b1c09cf has failed:

On branch mergify/bp/release/v0.48.x/pr-1303
Your branch is up to date with 'origin/release/v0.48.x'.

You are currently cherry-picking commit b1c09cf6c.
  (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/tx/factory.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.78%. Comparing base (8e8a284) to head (8cd3f6b).

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v0.48.x    #1307   +/-   ##
================================================
  Coverage            69.78%   69.78%           
================================================
  Files                  646      646           
  Lines                67526    67526           
================================================
  Hits                 47126    47126           
  Misses               18214    18214           
  Partials              2186     2186           

Files	Coverage Δ
client/tx/factory.go	60.50% <100.00%> (ø)