Move Trainable interface and add TrainingOptions

[mdekstrand]

This moves Trainable to a new lenskit.training module, and adds an options parameter of type TrainingOptions to provide options to the model training process, including a train-time RNG.

[codecov]

Codecov Report

Attention: Patch coverage is 95.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 89.96%. Comparing base (204dcee) to head (60e0972).
Report is 30 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/basic/history.py	80.00%	2 Missing ⚠️
lenskit/lenskit/pipeline/_impl.py	90.00%	2 Missing ⚠️
lenskit/lenskit/basic/candidates.py	85.71%	1 Missing ⚠️
lenskit/lenskit/basic/popularity.py	90.00%	1 Missing ⚠️
lenskit/lenskit/testing/_components.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #599   +/-   ##
=======================================
  Coverage   89.96%   89.96%           
=======================================
  Files         106      107    +1     
  Lines        6567     6587   +20     
=======================================
+ Hits         5908     5926   +18     
- Misses        659      661    +2     

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