Include component defaults in Optuna search space

[mdekstrand]

This updates the Optuna search to enqueue the component's default settings (from the component and its configuration) before it tries random points.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.55%. Comparing base (d930b51) to head (1bfcce7).

Files with missing lines	Patch %	Lines
src/lenskit/tuning/_optuna.py	86.36%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1145      +/-   ##
==========================================
- Coverage   89.56%   89.55%   -0.02%     
==========================================
  Files         256      256              
  Lines       17234    17274      +40     
==========================================
+ Hits        15435    15469      +34     
- Misses       1799     1805       +6     

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