Various enhancements in OpenAPI support

[bkoelman]

Backport of various improvements I identified while working on resource inheritance support. See the individual commit messages for details.

QUALITY CHECKLIST

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

[codecov]

Codecov Report

Attention: Patch coverage is 89.13043% with 25 lines in your changes missing coverage. Please review.

Project coverage is 90.38%. Comparing base (68cad58) to head (d10d51b).
Report is 10 commits behind head on openapi.

Files with missing lines	Patch %	Lines
...tors/Bodies/AtomicOperationsBodySchemaGenerator.cs	72.72%	0 Missing and 3 partials ⚠️
...ckle/SwaggerComponents/StringEnumOrderingFilter.cs	83.33%	1 Missing and 2 partials ⚠️
...mponents/AbstractAtomicOperationSchemaGenerator.cs	84.61%	1 Missing and 1 partial ⚠️
.../Components/AbstractResourceDataSchemaGenerator.cs	94.28%	1 Missing and 1 partial ⚠️
...erators/Components/DataContainerSchemaGenerator.cs	71.42%	1 Missing and 1 partial ⚠️
...SchemaGenerators/Components/DataSchemaGenerator.cs	96.87%	1 Missing and 1 partial ⚠️
...le/SwaggerComponents/ResourceFieldSchemaBuilder.cs	91.66%	1 Missing and 1 partial ⚠️
...Objects/Documents/CreateResourceRequestDocument.cs	0.00%	1 Missing ⚠️
...Objects/Documents/UpdateResourceRequestDocument.cs	0.00%	1 Missing ⚠️
...elationships/NullableToOneRelationshipInRequest.cs	0.00%	1 Missing ⚠️
... and 6 more

Additional details and impacted files

@@             Coverage Diff             @@
##           openapi    #1648      +/-   ##
===========================================
- Coverage    90.48%   90.38%   -0.10%     
===========================================
  Files          461      462       +1     
  Lines        13961    14027      +66     
  Branches      2172     2183      +11     
===========================================
+ Hits         12633    12679      +46     
- Misses         902      915      +13     
- Partials       426      433       +7     

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