OpenAPI: Show custom endpoints, fix content negotiation

[bkoelman]

This PR adds support for non-JSON:API endpoints (both Controllers and Minimal API endpoints), which now show up in SwaggerUI and generated clients. Custom action methods in JSON:API controllers are still hidden; a warning is logged now. It's currently unclear how that's supposed to work; please let us know what you expect from that by opening a new issue.

For JSON:API endpoints, the code that adds produces/consumes metadata was refactored, so it no longer influences ASP.NET routing. This fixes the bug where adding services.AddOpenApiForJsonApi() to your project changed the required media type for all requests.

Closes #1729.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

❌ Patch coverage is 96.39175% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.78%. Comparing base (8c59d1b) to head (07b69d6).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...uckle/JsonApiActionDescriptorCollectionProvider.cs	95.77%	2 Missing and 7 partials ⚠️
...JsonApiMetadata/JsonApiEndpointMetadataProvider.cs	90.90%	1 Missing and 1 partial ⚠️
....OpenApi.Swashbuckle/OpenApiContentTypeProvider.cs	87.50%	1 Missing and 1 partial ⚠️
...onApiMetadata/ActionMethods/JsonApiActionMethod.cs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1747      +/-   ##
==========================================
+ Coverage   90.75%   90.78%   +0.02%     
==========================================
  Files         468      474       +6     
  Lines       14662    14697      +35     
  Branches     2315     2319       +4     
==========================================
+ Hits        13306    13342      +36     
- Misses        920      921       +1     
+ Partials      436      434       -2     

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