pd control: add service-gc-safepoint command (#13311)

[ti-chi-bot]

This is an automated cherry-pick of #13311

What is changed, added or deleted? (Required)

Close #10690

service-gc-safepoint is introduced in tikv/pd#2797

~$ tiup ctl:v6.5.0 pd service-gc-safepoint
Starting component `ctl`: /home/.tiup/components/ctl/v6.5.0/ctl pd service-gc-safepoint
{
  "service_gc_safe_points": [
    {
      "service_id": "gc_worker",
      "expired_at": 9223372036854775807,
      "safe_point": 439923410637160448
    }
  ],
  "gc_safe_point": 0
}

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[qiancai]

/lgtm

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[Oreoxmt]

/lgtm

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Timeline:

• 2023-06-12 11:32:01.375251196 +0000 UTC m=+152070.830012556: ☑️ agreed by qiancai.
• 2023-06-12 11:41:24.184110896 +0000 UTC m=+152633.638872199: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

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