Support V2 TSO microservice discovery

[disksing]

Summary

• detect PD API service mode via GetClusterInfo
• discover TSO keyspace groups through tsopb.FindGroupByKeyspaceID
• open direct tsopb.TSO/Tso streams and cache timestamp oracles per keyspace id
• keep PD TSO as fallback for non-microservice deployments
• pass V2 keyspace ids into transactional timestamp paths

Validation

• cargo fmt
• cargo +nightly-2026-01-30 check
• cargo +nightly-2026-01-30 test --lib (58 tests)

Note: the pinned 1.84.1 toolchain did not reach project code in this local environment because fresh dependency resolution selected crates requiring edition2024; the repo ignores Cargo.lock.

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