feat(osmomath): BigDec Clone() and MulMut methods (backport #3678)

[mergify]

This is an automatic backport of pull request #3678 done by Mergify.
Cherry-pick of 1846a99 has failed:

On branch mergify/bp/v13.x/pr-3678
Your branch is up to date with 'origin/v13.x'.

You are currently cherry-picking commit 1846a99a.
  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	modified:   osmomath/decimal.go
	modified:   osmomath/decimal_test.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/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[ValarDragon]

Didn't realize the original PR wasn't to main

[ValarDragon]

EDIT: oh wait it was too main