chore(bigquery): send veneer header instead of gapic

[alvarowolfx]

Towards #2138

[gemini-code-assist]

Code Review

This pull request adds the httptest development dependency to the bigquery crate and configures the BigQuery client to include the XGoogApiClient extension, which sets the x-goog-api-client header to indicate a veneer library (gccl) with the package version. A new integration test has been added to verify that the correct header is sent and does not contain the gapic identifier. I have no feedback to provide as the changes are well-implemented and include appropriate test coverage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.18%. Comparing base (e295fe6) to head (84bacc0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6848      +/-   ##
==========================================
- Coverage   97.18%   97.18%   -0.01%     
==========================================
  Files         328      328              
  Lines      109962   110003      +41     
==========================================
+ Hits       106867   106902      +35     
- Misses       3095     3101       +6     

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[dbolduc]

Nice