Support NRT and MSV in `required` and `nullable`

[maurei]

This PR introduces improved support for required and nullable properties in OpenAPI schemas, by taking into account enabled/disabled nullable reference types and enabled/disabled model state validation. The mapping as outlined in the table in #1111 is now fully implemented.

Includes:

• Use the native NullabilityInfoContext for checking on nullability information rather than relying on Swashbuckles work-around (Supersedes #1167).
• Take into account if model state validation is enabled/disabled when generating OpenAPI schemas.
• Small tweak in OpenApiTestContext to allow for retrieving a swagger file without having to write it to disk.

Closes #1111

[codecov[bot]]

Codecov Report

Merging #1185 (0dd49b0) into openapi (16a0026) will increase coverage by 0.06%. The diff coverage is 95.77%.

@@             Coverage Diff             @@
##           openapi    #1185      +/-   ##
===========================================
+ Coverage    92.65%   92.72%   +0.06%     
===========================================
  Files          329      329              
  Lines         9863     9941      +78     
===========================================
+ Hits          9139     9218      +79     
+ Misses         724      723       -1     

Files	Coverage Δ
...tNetCore.OpenApi.Client/Exceptions/ApiException.cs	87.50% <ø> (ø)
...piDotNetCore.OpenApi/ActionDescriptorExtensions.cs	100.00% <100.00%> (ø)
...enApi/JsonApiActionDescriptorCollectionProvider.cs	96.00% <100.00%> (+0.05%)	:arrow_up:
...JsonApiMetadata/JsonApiEndpointMetadataProvider.cs	96.00% <100.00%> (+1.00%)	:arrow_up:
...i/JsonApiMetadata/NonPrimaryDocumentTypeFactory.cs	100.00% <100.00%> (ø)
...OpenApi/JsonApiMetadata/RelationshipTypeFactory.cs	100.00% <100.00%> (ø)
...piDotNetCore.OpenApi/JsonApiOperationIdSelector.cs	92.45% <ø> (ø)
...iDotNetCore.OpenApi/ServiceCollectionExtensions.cs	98.55% <100.00%> (+0.08%)	:arrow_up:
...enApi/SwaggerComponents/CachingSwaggerGenerator.cs	100.00% <ø> (ø)
...penApi/SwaggerComponents/EndpointOrderingFilter.cs	100.00% <100.00%> (ø)
... and 7 more

... and 1 file with indirect coverage changes

[bkoelman]

I took the liberty to merge in #1186, I hope you don't mind. However, I was surprised that running the tests changed two swagger.g.json files on disk, without any of the tests failing. That doesn't seem right: when an OAS file changes unexpectedly, something definitely should fail.

[maurei]

The review items I've reacted to are processed in my latest commit. The other ones are related to something I'm still investigating and will follow

[maurei]

I took the liberty to merge in #1186, I hope you don't mind. However, I was surprised that running the tests changed two swagger.g.json files on disk, without any of the tests failing. That doesn't seem right: when an OAS file changes unexpectedly, something definitely should fail.

Haven't been able to reproduce this. Compared two fresh clones of this branch where in only one of them I ran the tests

[maurei]

@bkoelman there are tests failing due to timeouts unrelated to this PR. It seems to happens during the test cleanup phase which you recently worked on. Do you have any idea what could be causing this?

See for example https://ci.appveyor.com/project/json-api-dotnet/jsonapidotnetcore/builds/45792318/job/l2grxineggemqslq#L317