expression: Support push function unix_timestamp, concat down to TiFlash. (#25083)

[ti-srebot]

cherry-pick #25083 to release-5.0
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/25146

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

git push git@github.com:ti-srebot/tidb.git pr/25146:release-5.0-6a1ebeb020d2

---

What problem does this PR solve?

Issue Number: pingcap/tiflash#1940, pingcap/tiflash#1939

Problem Summary:

Function: push unix_timestamp, concat to TiFlash.

What is changed and how it works?

Proposal: xxx

What's Changed:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch: no need

Check List

Tests

• Unit test
• Integration test

Side effects

Release note

• Support push function unix_timestamp, concat down to TiFlash.

[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]

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

[LittleFall]

should be closed by #25564

[lzmhhh123]

Already cherry-picked.