Add support for x-enumDescriptions extension

[federicobond]

This adds support for the x-enumDescriptions OpenAPI extension field as documented here:
https://redocly.com/docs/realm/author/reference/openapi-extensions/x-enum-descriptions

Redoc and Scalar support this extension and provide a better UX than text-based descriptions.

The change is fully backward-compatible. I added a new boolean setting ENUM_GENERATE_X_ENUM_DESCRIPTIONS to enable this feature. Let me know if this makes sense for you.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (5734744) to head (3c14118).
⚠️ Report is 126 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1359      +/-   ##
==========================================
+ Coverage   98.42%   98.57%   +0.15%     
==========================================
  Files          74       74              
  Lines        9139     9146       +7     
==========================================
+ Hits         8995     9016      +21     
+ Misses        144      130      -14     

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