feat: Add Categories on the Python side

[orlp]

This is not yet documented and officially part of the API, but we really needed this for testing already.

[codecov]

Codecov Report

Attention: Patch coverage is 84.37500% with 25 lines in your changes missing coverage. Please review.

Project coverage is 81.45%. Comparing base (551ff1f) to head (b13ac7c).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/datatypes/classes.py	76.13%	16 Missing and 5 partials ⚠️
crates/polars-dtype/src/categorical/mod.rs	76.92%	3 Missing ⚠️
crates/polars-python/src/conversion/categorical.rs	97.82%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23543      +/-   ##
==========================================
- Coverage   81.51%   81.45%   -0.06%     
==========================================
  Files        1635     1640       +5     
  Lines      221735   222210     +475     
  Branches     2813     2835      +22     
==========================================
+ Hits       180748   181010     +262     
- Misses      40298    40501     +203     
- Partials      689      699      +10     

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