Implement collectible warnings and warning collection for batch inference

[mdekstrand]

This adds a warning collection / counting capability, and enables it for batch inference.

Closes #1113.

[codecov]

Codecov Report

❌ Patch coverage is 76.19048% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.57%. Comparing base (5815b92) to head (c364df6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/logging/_warnings.py	72.54%	14 Missing ⚠️
src/lenskit/knn/user.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1168      +/-   ##
==========================================
- Coverage   89.61%   89.57%   -0.05%     
==========================================
  Files         261      262       +1     
  Lines       17329    17385      +56     
==========================================
+ Hits        15530    15572      +42     
- Misses       1799     1813      +14     

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