Update kvproto dependency to APIV3 definitions

The kvproto APIV3 branch changes keyspace-related protobuf fields to oneofs. PD needs to update the dependency and migrate TSO, keyspace, GC safe point, HTTP, and pd-ctl call sites to the generated oneof APIs.

This issue tracks the downstream compatibility update.

Start by updating the kvproto dependency to the APIV3 definitions and inspect the generated oneof APIs. Trace the affected TSO, keyspace, GC safe point, HTTP, and pd-ctl call sites, then verify that all downstream compatibility paths build and their existing tests pass.

Written by the indexing model from the issue text.