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
• Add implementation new RPC interfaces. (may need to further breakdown into muitiple subtasks later)
• Implement the new set of methods on pd.Client interface type.
• New pd-ctl commands for replacing the current service-gc-safepoint command.
• Mark the old API deprecated.