feat: basic schema discriminator support

[danielgtaylor]

Add basic support for the OpenAPI 3.1 discriminator object in schemas. https://spec.openapis.org/oas/v3.1.0#discriminator-object

Fixes #384

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.72%. Comparing base (da22a6c) to head (33506ff).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
+ Coverage   92.71%   92.72%   +0.01%     
==========================================
  Files          21       21              
  Lines        3540     3546       +6     
==========================================
+ Hits         3282     3288       +6     
  Misses        220      220              
  Partials       38       38              

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