br/cmd: implement the operator pause-gc-and-schedulers command (#43562)

[ti-chi-bot]

This is an automated cherry-pick of #43562

What problem does this PR solve?

Issue Number: ref #43559
Problem Summary:
This PR implements the new (hidden) command "br operator pause-gc", which blocks the shell and then pause the global GC safepoint.
There are two flags for this command:
--safepoint or -t, the GC safepoint to be kept, default current resolved ts.
--ttl or -i, how long the service safe point lives, default 5 mins.

What is changed and how it works?

This PR just reused the StartServiceGcSafePointKeeper.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  (It is not easy to auto test this PR.)
  Firstly, we paused the safepoint.

The metrics shows that the safepoint has been kept.

This test also applies to the schedulers.

• No code

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• YuJuncen

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[YuJuncen]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 523f6a1

[hawkingrei]

/retest