tso: enhance timestamp persistency with strong leader consistency (#9171)

[ti-chi-bot]

This is an automated cherry-pick of #9171

What problem does this PR solve?

Issue Number: close #9157.

What is changed and how does it work?

This commit enhances the timestamp storage mechanism by adding a leadership validation check when saving TSO timestamps to etcd.
This prevents a former leader from incorrectly persisting timestamps after losing leadership.

Check List

Tests

• Unit test
• Integration test

Release note

Fix a bug that could cause the former PD leader to incorrectly persist timestamps after losing its leadership.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
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[ti-chi-bot]

@JmPotato This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci/build	bc658ae	link	true	/test build

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