impl(bigquery): multiplexing option on writer builder

[dbolduc]

This PR adds a fluent setter on the writer builder to enable multiplexing and plumbs the options from the client builder into the client into the writer builder into the writer.

All of this is pub(crate) for now. The next PR will make them pub.

Note: we will need separate connection pools for arrow vs. proto transports, so I am only implementing arrow right now.

Note: we discussed changing the signature of .arrow().default() in the API review. I am still going to look into that, but after I get this feature done. (There is one PR after this).

Towards #6765

[gemini-code-assist]

Code Review

This pull request introduces multiplexing support for the BigQuery Arrow writer by updating WriterBuilder to accept a shared StreamPool and adding a with_multiplexing option. The Write client now manages and passes this shared pool to the builder. Feedback suggests introducing a helper function in the test module to eliminate repetitive boilerplate code when creating WriterBuilder instances across various test cases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.02%. Comparing base (63e9c2a) to head (60abfea).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6811   +/-   ##
=======================================
  Coverage   97.02%   97.02%           
=======================================
  Files         326      326           
  Lines      107548   107585   +37     
=======================================
+ Hits       104350   104388   +38     
+ Misses       3198     3197    -1     

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