Refactor GC related PD APIs

[MyonKeminta]

Development Task

This is the majority part of the refactoring task pingcap/tidb#58720.

A set of new APIs for GC will be implemented which is expected to provide a more comprehensible semantics and recude potential misuses. This will also be the basis for implementing a planned feature name protected snapshots.

Subtasks

Note that protected snapshots won't be included in this phase and will be another separated work after finishing the refactoring.

• Add new RPC calls to kvproto Add new GC API pingcap/kvproto#1288
• Expose the if-then-else transaction interface of etcd to kv.Base layer, parallel to the existing kv.Base.RunInTxn.
• Add implementation new RPC interfaces. (may need to further breakdown into muitiple subtasks later)
  • Basic API set.
  • Support controlling disabling physical GC operations on TiKV (required by BR)
  • Support watching global GC state changes (including all keysapces; maybe required by keyspace-level-GC for running GC in TiKV)
• Implement the new set of methods on pd.Client interface type.
  • Basic API set
  • Support controlling disabling physical GC operations on TiKV
• New pd-ctl commands gc-state for replacing the current service-gc-safepoint command.
• Mark the old API deprecated.