tonic-xds: Add A32 circuit breaking limiter

[mingley]

Motivation

gRFC A32 circuit breaking is parsed by tonic-xds, but requests are not yet limited. This splits out the reusable enforcement primitive first so the request-lifetime behavior can be reviewed independently before production CDS/channel wiring lands.

This is part of #2444, which tracks adding xDS support to Tonic and gRPC.

Solution

• Add a client::circuit_breaking Tower layer that enforces max_requests with process-global counters keyed by cluster and EDS service name.
• Hold permits for the full response-body lifetime, including streaming responses and dropped response futures.
• Return gRPC statuses for circuit-breaking and missing routing decisions, keep local drops out of retry policy, and track drop counts for future LRS export.
• Add unit coverage for rejection, permit release, cancellation, retry behavior, EDS-service-name keying, and counter cleanup.

Testing Done

• cargo fmt --all --check
• cargo check -p tonic-xds --all-features
• cargo test -p tonic-xds circuit_breaking --lib

[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