fix: Expose FieldsMapper

[stijnherfst]

This type is needed by pyo3-polars but was not expose in the API when the file schema.rs was moved.

[github-actions]

The uncompressed lib size after this PR is 41.5473 MB.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.61%. Comparing base (96dd613) to head (ad84186).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23232      +/-   ##
==========================================
- Coverage   80.62%   80.61%   -0.01%     
==========================================
  Files        1639     1639              
  Lines      220027   220027              
  Branches     2814     2814              
==========================================
- Hits       177387   177377      -10     
- Misses      41974    41984      +10     
  Partials      666      666              

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