refactor(bigquery): genericize DefaultWriter

[dbolduc]

This PR refactors the arrow::DefaultWriter and proto::DefaultWriter to share a common implementation.

The crate::write:::DefaultWriter<F> is ultimately the type we want to expose to applications. By the end of #6855 we will delete the other versions.

I decided we should expose the final types now because the docs are unusable if we try to hide them:

https://screenshot-v2.corp.google.com/1c0t3ta9teocg

vs.

https://screenshot-v2.corp.google.com/2fm1nq03p61g0

Towards #6855, #6762

[gemini-code-assist]

Code Review

This pull request refactors DefaultWriter to be generic over the data format (DefaultWriter<F>), moving it to a shared write module to support both Arrow and Proto formats. Feedback on these changes includes correcting the import paths in the tests module of default.rs to use super:: instead of super::super::, and deriving Clone on DefaultWriter to facilitate sharing across asynchronous tasks without introducing double indirection.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.18%. Comparing base (9925abe) to head (d86d1a5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6860      +/-   ##
==========================================
- Coverage   97.18%   97.18%   -0.01%     
==========================================
  Files         331      330       -1     
  Lines      110482   110394      -88     
==========================================
- Hits       107373   107283      -90     
- Misses       3109     3111       +2     

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