gc: make GC state reads respect request cancellation

[wfxr]

What problem does this PR solve?

Issue Number: Close #11131

GC state read requests continue waiting for the internal etcd operation timeout after the caller has canceled the gRPC or HTTP request. Because the manager holds its read lock while loading GC state, the abandoned request can also delay leadership transitions and other operations that require the write lock.

What is changed and how does it work?

Propagate request contexts through GC state manager reads, GC state
transactions, and etcd-backed Range and Txn operations so canceled requests
stop blocked storage operations promptly.

Add opt-in context-aware storage capabilities while preserving the existing
contextless interfaces for compatibility. Backends that do not implement the
new capabilities return an explicit error instead of silently falling back to
non-cancelable I/O.

Preserve the manager's existing locking, cache, transaction, and singleflight
semantics. Canceled reads release the manager read lock and do not populate the
current keyspace cache.

Return standard gRPC cancellation and deadline status codes only when the RPC
request context itself has ended. Internal etcd operation timeouts retain the
existing protobuf error-header behavior.

Check List

Tests

• Unit test

Release note

Fix an issue where canceled GC state read requests could continue waiting for the internal etcd timeout and delay PD leadership transitions.

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