impl(bigquery): allow retry/backoff policy override and use default policies

[alvarowolfx]

Towards #6218

[gemini-code-assist]

Code Review

This pull request introduces with_retry_policy and with_backoff_policy builder methods to the BigQuery ClientBuilder, enabling custom retry and backoff configurations. It also updates the client to fall back on default policies when none are specified, cleans up obsolete #[allow(dead_code)] attributes, and adds unit tests. The feedback recommends removing an unused std::time::Duration import from the new doctest to avoid potential compilation warnings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.57%. Comparing base (d6577c1) to head (99de16b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6639      +/-   ##
==========================================
+ Coverage   96.55%   96.57%   +0.01%     
==========================================
  Files         306      306              
  Lines       94212    94236      +24     
==========================================
+ Hits        90969    91005      +36     
+ Misses       3243     3231      -12     

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