Add .list() and .value() methods to entity attributes

[mdekstrand]

This adds .list() and .value() methods to entity attributes, to improve convenience of easy access.

Also fixes some type-casting bugs with vocabulary ID types.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.48%. Comparing base (adb9361) to head (d1d7257).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/data/_items.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1088      +/-   ##
==========================================
+ Coverage   89.45%   89.48%   +0.03%     
==========================================
  Files         231      231              
  Lines       15983    15994      +11     
==========================================
+ Hits        14298    14313      +15     
+ Misses       1685     1681       -4     

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