feat(python): Add Schema class

[stinodego]

Ref #16328

Changes

• Introduce a new Schema class. It is currently basically just an OrderedDict, but has some additional methods:
  • names
  • dtypes
  • len
• Update various schema methods to return this new Schema class instead of an OrderedDict.

Because it inherits OrderedDict, it can also be used as input for schema arguments anywhere in the API, e.g. the DataFrame constructor, read_csv, etc.

I will expand the Schema class in the future to be a bit more user friendly for direct use. For now it's already useful as a returned object from the upcoming LazyFrame.collect_schema method.

[codecov]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.40%. Comparing base (47d19f3) to head (63a2cd0).

Files	Patch %	Lines
py-polars/polars/series/struct.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16873      +/-   ##
==========================================
- Coverage   81.41%   81.40%   -0.01%     
==========================================
  Files        1425     1426       +1     
  Lines      187964   187980      +16     
  Branches     2704     2704              
==========================================
+ Hits       153022   153024       +2     
- Misses      34445    34459      +14     
  Partials      497      497              

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