add API V3 routing to Rust client

[disksing]

Summary

• Add API V3 client context identity support and physical-key-only PD routing helpers.
• Keep TiKV KV RPC request keys as user keys while route cache lookup uses V3 physical keys.
• Add global keyspace name lookup for DB9-style globally unique keyspace names.

Validation

• cargo +stable check
• cargo +stable test test_api_v3_ -- --nocapture
• cargo +stable test test_apiv2_no_prefix_api_version -- --nocapture

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