impl(bigquery): default write retry policy

[dbolduc]

Define the default retry policy for the Write client. We will eventually expose this.

Note that all appends are idempotent as the service offers at-least-once semantics. The policy does not need to check it, it can assume.

Part of the work for #6355

[gemini-code-assist]

Code Review

This pull request introduces a new retry policy for the BigQuery Storage Write API, along with comprehensive unit tests. The reviewer suggests renaming the RetryableErrors struct to WriteRetryPolicy to better align with standard Rust naming conventions and maintain consistency across the SDK.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.02%. Comparing base (27f8c1e) to head (d8d12ca).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6813      +/-   ##
==========================================
- Coverage   97.02%   97.02%   -0.01%     
==========================================
  Files         326      327       +1     
  Lines      107548   107624      +76     
==========================================
+ Hits       104352   104425      +73     
- Misses       3196     3199       +3     

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[alvarowolfx]

Just make sure to add Code::Cancelled to the retry list