feat(python): Expose unity catalog dataclasses and type aliases

[nameexhaustion]

This exposes the dataclasses and some type aliases (e.g. TableType) to Python (importable from polars.catalog.unity), as well as the reference guide:

The existing reference guide entries are also moved to a "Unity Catalog" subsection in the reference guide.

Otherwise, the rest of this diff in this PR would be moving code, mainly splitting py-polars/polars/catalog.py into separate files:

py-polars/polars/catalog/
├── __init__.py
└── unity
    ├── __init__.py
    ├── client.py
    └── models.py

[codecov]

Codecov Report

Attention: Patch coverage is 14.53155% with 447 lines in your changes missing coverage. Please review.

Project coverage is 79.27%. Comparing base (db712e4) to head (dd275b3).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-python/src/catalog/unity.rs	3.12%	434 Missing ⚠️
py-polars/polars/catalog/unity/models.py	80.59%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21046      +/-   ##
==========================================
- Coverage   79.28%   79.27%   -0.01%     
==========================================
  Files        1583     1585       +2     
  Lines      225549   225557       +8     
  Branches     2586     2586              
==========================================
+ Hits       178816   178817       +1     
- Misses      46143    46150       +7     
  Partials      590      590              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.