Track API V3 keyspace identity support in PD

[disksing]

Enhancement Task

Track the PD-side work needed to support API V3 keyspace identity.

The original API V3 PR is being split into smaller reviewable PRs. The split
work covers:

• Cleaning up keyspace tests that depended on mock kernel-mode failpoints.
• Updating kvproto usage after keyspace ID fields move to oneof wrappers.
• Adding PD client APIs and internal plumbing for API V3 keyspace identity.
• Routing TSO requests and TSO service discovery by API V3 keyspace identity.

Related PRs:

• Add API V3 keyspace identity to PD client #10984
• test: clean up keyspace kernel mode tests #10987