ttl, variables: add system variables for ttl job schedule (#39727)

[ti-chi-bot]

This is an automated cherry-pick of #39727

What problem does this PR solve?

Issue Number: close #39725

Problem Summary:

Add the following system variable for ttl job schedule:

• tidb_ttl_job_run_interval
• tidb_ttl_job_schedule_window_start_time
• tidb_ttl_job_schedule_window_end_time
• tidb_ttl_scan_worker_count
• tidb_ttl_delete_worker_count

It also removes the TTLFeatureGate

What is changed and how it works?

These global variables (except worker counts) are all tracked by an atomic variable, and set/get through the hook. The scan/delete worker count are set/get through scaling the workers in the ttl job manager.

This PR is based on #39472

Check List

Tests

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

Release note

Add system variables `tidb_ttl_job_run_interval`, `tidb_ttl_job_schedule_window_start_time`, `tidb_ttl_job_schedule_window_end_time`, `tidb_ttl_scan_worker_count`, `tidb_ttl_delete_worker_count` to control the ttl job management

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• YangKeao
• bb7133

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[YangKeao]

LGTM

[YangKeao]

/run-check_dev_2

[YangKeao]

/run-build

[bb7133]

LGTM

[YangKeao]

/run-unit-test

[YangKeao]

Oops. To pass the unit-test, it requires #39767

[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: 73e519b

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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