server: support decoding prepared string args to character_set_client (#30723)

[ti-srebot]

cherry-pick #30723 to release-5.3
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/30735

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/30735:release-5.3-04a9618f5ccc

---

What problem does this PR solve?

Fix #30722.

Problem Summary:

Previously, the command ComStmtExecute is not aware of @@character_set_client.

What is changed and how it works?

This PR support

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@tangenta you're already a collaborator in bot's repo.

[tangenta]

/run-build-arm64 comment=true

[purelind]

/run-build-arm64 comment=true

[sre-bot]

download tidb binary(linux arm64) at http://fileserver.pingcap.net/download/builds/pingcap/test/tidb/648e3da809a65af58c0d06bf1851386e0c1c70b6/centos7/tidb-linux-arm64.tar.gz

[guo-shaoge]

/run-mysql-test
@tangenta please remember to merge this. A new version is about to release. Thanks!

[tangenta]

The feature GBK charset is not fully supported in release 5.3.