Refactor embedding size configuration for consistency + exponential support

[mdekstrand]

This adds the idea of “common configuration mixins”, to define common behavior across multiple configuration classes, and defines one for embedding_size, that has the useful behavior of supporting an embedding_size_exp alias to specify the embedding size as a power of 2.

[codecov]

Codecov Report

❌ Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.83%. Comparing base (763dbb6) to head (72b8064).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/logging/progress/_rich.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #851   +/-   ##
=======================================
  Coverage   89.82%   89.83%           
=======================================
  Files         157      158    +1     
  Lines       11193    11202    +9     
=======================================
+ Hits        10054    10063    +9     
  Misses       1139     1139           

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