br/backup_ebs: added flag operator-paused-gc-and-scheduler.

[YuJuncen]

What problem does this PR solve?

Issue Number: ref #43559

Problem Summary:
After #43562, we get the ability of pausing GC and schedulers in an external pod. But the backup command still tries to pause them, which is not really effective and will bring the ABA problem. (Say, some of BRs think the state of "no scheduler exists" is the original state of the cluster.)

What is changed and how it works?

This PR added a new hidden flag --operator-paused-gc-and-scheduler. Once it set, the client will no longer try to pause scheduler (Should / Is it possible to further check whether they are really paused?) and start backup directly after the pending split done.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  Run command:

br_ebs_backup_mocking_wait_snapshot_duration=800s ~/br-operator backup full --type aws-ebs --skip-aws -u http://upd-1:2379 -s noop://

The schedulers are paused.

Run command:

 br_ebs_backup_mocking_wait_snapshot_duration=800s ~/br-operator backup full --type aws-ebs --skip-aws --operator-paused-gc-and-scheduler -u http://upd-1:2379 -s noop://

The schedulers are not paused.

Release note

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• 3pointer
• BornChanger

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[YuJuncen]

/retest

[3pointer]

/merge

[ti-chi-bot]

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

Commit hash: 290d515

[YuJuncen]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #43926.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #43927.