pdpb: add store scheduling state to GetStore response

[LykxSassinator]

Summary

Issue: ref tikv/client-go#2012

• Add StoreSchedulingState to GetStoreResponse for PD-owned, per-store scheduling decisions used by clients during replica selection.
• Expose evicted_as_slow_store, aligned with PD's recovery-time-based slow-store eviction decision and distinct from local health observations or store lifecycle metadata.
• Keep the dynamic state scoped to GetStoreResponse, avoiding pollution of the shared metapb.Store metadata path.
• Document absent state as unknown rather than an implicit false, and reserve the message for independent future constraints such as slow-trend or stopping-store eviction.
• Regenerate Go protobuf bindings and update proto.lock.

Tests

• Manual build check

[ti-chi-bot]

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