Implement usable pipeline configuration files and CLI support

[mdekstrand]

This adds support for human-authorable configuration files (#825) and support configuration files in the CLI (#826).

Closes #825. Closes #826.

[codecov]

Codecov Report

❌ Patch coverage is 90.34483% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.94%. Comparing base (895f3d1) to head (408dfbc).
⚠️ Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/pipeline/builder.py	85.10%	7 Missing ⚠️
src/lenskit/cli/train.py	60.00%	4 Missing ⚠️
src/lenskit/config/__init__.py	91.30%	2 Missing ⚠️
src/lenskit/pipeline/components.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #831      +/-   ##
==========================================
+ Coverage   89.82%   89.94%   +0.11%     
==========================================
  Files         153      153              
  Lines       10654    10757     +103     
==========================================
+ Hits         9570     9675     +105     
+ Misses       1084     1082       -2     

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