Add OpenAPI support for custom JSON:API action methods

[bkoelman]

Closes #1066.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• Documentation updated

[codecov]

Codecov Report

❌ Patch coverage is 92.06897% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.05%. Comparing base (5f85f93) to head (9a81018).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...JsonApiMetadata/JsonApiEndpointMetadataProvider.cs	87.33%	11 Missing and 8 partials ⚠️
...uckle/JsonApiActionDescriptorCollectionProvider.cs	95.45%	1 Missing and 1 partial ⚠️
...adata/ActionMethods/BuiltinResourceActionMethod.cs	66.66%	1 Missing ⚠️
...re/Queries/Expressions/SparseFieldSetExpression.cs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1787      +/-   ##
==========================================
+ Coverage   92.01%   92.05%   +0.03%     
==========================================
  Files         475      473       -2     
  Lines       14730    14818      +88     
  Branches     2339     2361      +22     
==========================================
+ Hits        13554    13640      +86     
+ Misses        772      770       -2     
- Partials      404      408       +4     

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