feat(tonic-xds): parse A32 circuit breaking config

[mingley]

Motivation

This is the first scaffold PR for gRFC A32 circuit breaking under #2444. It starts carrying the CDS circuit_breakers.thresholds[].max_requests value through the validated ClusterResource without changing request routing or load-balancing behavior yet.

Solution

Adds a small CircuitBreakingConfig resource helper that:

• defaults max_requests to 1024 when CDS circuit breakers are absent or no DEFAULT threshold is configured
• selects the first RoutingPriority::Default threshold, matching Envoy/gRFC behavior
• parses only max_requests; all other circuit-breaker fields remain ignored for this scaffold

Enforcement is intentionally left to a follow-up PR so permit lifetime can be handled correctly for streaming RPC response bodies.

Testing Done

• cargo fmt --all --check
• cargo test -p tonic-xds xds::resource
• cargo check -p tonic-xds --all-features

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mingley / name: Michael Ingley (10e741c, 8e5ba25)

[YutaoMa]

Same comment as in #2701, let's avoid validating config end to end until the feature support is added: you can do this either by detaching the wiring from Cluster config layer, or implement logic first and then come back to this PR.

[mingley]

Same comment as in #2701, let's avoid validating config end to end until the feature support is added: you can do this either by detaching the wiring from Cluster config layer, or implement logic first and then come back to this PR.

addressed in 9afebba