tonic-xds: Add A32 circuit breaking limiter

[mingley]

Summary

gRFC A32 circuit-breaking configuration is parsed by tonic-xds, but request enforcement is not yet in the channel path. This PR adds the reusable limiter primitive first so its request-lifetime and state-management behavior can be reviewed independently before production CDS/channel wiring lands. It is part of #2444.

• Enforce max_requests with process-global in-flight counters keyed by cluster and EDS service name, while retaining per-channel drop accounting for future LRS integration (36b03763, 4a20f392, c6b89daa).
• Admit calls only after the wrapped per-cluster service is ready, so queued work does not consume capacity, and mark local UNAVAILABLE drops so gRPC retry policy does not retry them (c6b89daa).
• Hold permits through the full response-body lifetime and release them on trailers, body errors, completion, response-future cancellation, or response drop (36b03763, c6b89daa).
• Apply live limit changes without resetting in-flight counts and clean up counters across EDS service-name changes, config removal/recreation, and channel teardown (c4559d3e, 4a20f392, c6b89daa).
• Cover the default 1024-request boundary, process-wide sharing, per-channel drop attribution, retry behavior, readiness, cancellation, and counter cleanup (c6b89daa).

Testing Done

• cargo fmt --all --check
• cargo clippy -p tonic-xds --lib --all-features -- -D warnings - passed
• cargo test -p tonic-xds circuit_breaking --lib - 23 passed, 0 failed
• cargo test -p tonic-xds --all-features - 389 unit tests and 4 doc tests passed
• gh pr checks 2711 --repo grpc/grpc-rust - all non-skipped PR checks passed

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mingley / name: Michael Ingley (4609832)

[YutaoMa]

For PRs to tonic-xds, mention that in the PR title for better searchability.

[mingley]

For PRs to tonic-xds, mention that in the PR title for better searchability.

updated PR description to track #2444 and also updated title to include tonic-xds