feat(api): extra_settings for SettingsReader

[henryiii]

Pulling out the API change from #637.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 86.23%. Comparing base (276bb17) to head (f5c8072).

Files	Patch %	Lines
...cikit_build_core/settings/skbuild_read_settings.py	80.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #697      +/-   ##
==========================================
- Coverage   86.30%   86.23%   -0.07%     
==========================================
  Files          64       64              
  Lines        3315     3321       +6     
==========================================
+ Hits         2861     2864       +3     
- Misses        454      457       +3     

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