chore(spanner): core implementation for a dynamic channel pool

[olavloite]

Introduce modular channel pooling supporting static fixed-size pools and dynamic load-based scaling with Power of Two Least Busy (P2C) selection, parallel channel priming, and transaction affinity routing.

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive dynamic channel pooling implementation for the Spanner client. It includes transaction channel affinity management, static and dynamic pool configurations, channel entry lifecycle tracking with RAII guards, a core channel pool engine utilizing Power of Two Least Busy selection, and background scaling workers for autonomous scale-up, scale-down, and priming. I have no feedback to provide as there are no review comments to assess.

[codecov]

Codecov Report

❌ Patch coverage is 78.41270% with 272 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.13%. Comparing base (102298f) to head (3c4f256).

Files with missing lines	Patch %	Lines
src/spanner/src/channel_pool/scaler.rs	56.29%	191 Missing ⚠️
src/spanner/src/channel_pool/pool.rs	90.41%	32 Missing ⚠️
src/spanner/src/channel_pool/config.rs	89.52%	22 Missing ⚠️
src/spanner/src/channel_pool/entry.rs	90.45%	21 Missing ⚠️
src/spanner/src/channel_pool/affinity.rs	89.83%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6522      +/-   ##
==========================================
- Coverage   96.39%   96.13%   -0.26%     
==========================================
  Files         301      306       +5     
  Lines       84762    86022    +1260     
==========================================
+ Hits        81706    82697     +991     
- Misses       3056     3325     +269     

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