impl(bigquery): introduce stream dispatcher

[dbolduc]

Add a type that handles interactions with the stream pool. Eventually it will handle retries for individual writes.

It caches the StreamEntry (basically a handle on a gRPC stream) so that pool resizing is independent of writing on valid streams.

Eventually, we can even rebalance writers to other streams by swapping out the Dispatcher's ArcSwap<StreamEntry> also without slowing down the hot path.

Towards #5831

[gemini-code-assist]

Code Review

This pull request introduces a new Dispatcher struct in the BigQuery write module to efficiently dispatch writes to a stream pool using arc-swap for atomic stream loading and replacement on transient errors. The feedback suggests replacing the deprecated compare_and_swap method with compare_exchange to avoid deprecation warnings, and proactively evicting closed streams during stream selection to optimize the recovery process.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.95%. Comparing base (66a179e) to head (b4f03c9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6743      +/-   ##
==========================================
+ Coverage   96.89%   96.95%   +0.06%     
==========================================
  Files         319      320       +1     
  Lines      105574   105722     +148     
==========================================
+ Hits       102292   102506     +214     
+ Misses       3282     3216      -66     

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