feat(codec): add collections value codec for protov2 APIs. (backport #17042)

[mergify]

This is an automatic backport of pull request #17042 done by Mergify.
Cherry-pick of 3d15233 has failed:

On branch mergify/bp/release/v0.50.x/pr-17042
Your branch is up to date with 'origin/release/v0.50.x'.

You are currently cherry-picking commit 3d1523357.
  (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:   codec/collections.go
	modified:   codec/collections_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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[julienrbrt]

After this and #17066, I'll tag v0.50 beta.