impl(bigquery): add client builder

[alvarowolfx]

Towards #5844

[gemini-code-assist]

Code Review

This pull request introduces a high-level BigQuery client and its associated ClientBuilder to configure service endpoints, credentials, and retry policies. The feedback recommends removing unused code and configurations, specifically the #[allow(dead_code)] attribute on job_service and the unused storage_endpoint field, to align with repository guidelines against introducing incomplete features.

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.88%. Comparing base (8603f7d) to head (53c8e65).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bigquery/src/client.rs	85.71%	5 Missing ⚠️
src/bigquery/src/client_builder.rs	93.87%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5869      +/-   ##
==========================================
- Coverage   97.89%   97.88%   -0.02%     
==========================================
  Files         227      229       +2     
  Lines       57992    58076      +84     
==========================================
+ Hits        56774    56849      +75     
- Misses       1218     1227       +9     

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