Add API V3 keyspace identity support

[disksing]

Summary

• add API V3 keyspace identity plumbing for txnkv clients
• add a V3 codec that uses physical prefixes for PD region routing while sending user keys plus KeyspaceIdentity to TiKV RPCs
• expose V3 codec/PD constructors and focused codec tests

Validation

• go test ./internal/apicodec ./internal/locate ./txnkv

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✨ Finishing Touches

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