OpenAPI: Error responses

[bkoelman]

This PR adds OpenAPI support for error response schemas, compliant with the JSON:API errors structure.

Example in SwaggerUI:

Example usage with NSwag generated C# client:

Example usage with Kiota generated C# client:

Closes #1057.

QUALITY CHECKLIST

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

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (f7f8f8b) 90.67% compared to head (0cd9754) 90.70%.

Files	Patch %	Lines
...ApiDotNetCore.OpenApi/OpenApiEndpointConvention.cs	93.22%	2 Missing and 2 partials ⚠️
...rc/JsonApiDotNetCore.OpenApi.Client/ApiResponse.cs	50.00%	1 Missing ⚠️
...tNetCore.OpenApi.Client/Exceptions/ApiException.cs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           openapi    #1460      +/-   ##
===========================================
+ Coverage    90.67%   90.70%   +0.02%     
===========================================
  Files          390      390              
  Lines        12735    12785      +50     
  Branches      2061     2057       -4     
===========================================
+ Hits         11547    11596      +49     
- Misses         772      774       +2     
+ Partials       416      415       -1     

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