chore(gax): support request attempt interception via options extensions

[olavloite]

Note: This is an alternative implementation of the same feature as #5966

Allow mutating request headers on every RPC attempt (including retries) by checking for a closure extension inside RequestOptions.

If an Arc<dyn Fn(&mut HeaderMap, u32) + Send + Sync> is present in the request options extensions, it is executed before each request attempt with the 1-based attempt number. This enables downstream clients to inject and attempt-specific request headers without modifying the public API of google-cloud-gax[-internal] or introducing new public traits.

[gemini-code-assist]

Code Review

This pull request introduces an attempt interceptor mechanism for gRPC requests, allowing custom headers to be injected or modified per attempt, including retries. It also adds integration tests to verify the interceptor's behavior. The review feedback suggests extending this interceptor support to bidirectional streaming requests to ensure consistent behavior across all RPC types.

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.72%. Comparing base (ac6f825) to head (5de816e).

Files with missing lines	Patch %	Lines
src/gax-internal/src/grpc.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6014      +/-   ##
==========================================
- Coverage   97.73%   97.72%   -0.01%     
==========================================
  Files         244      244              
  Lines       61161    61170       +9     
==========================================
+ Hits        59776    59781       +5     
- Misses       1385     1389       +4     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.