OpenAPI: Add type hints on 'id' parameters and properties

[bkoelman]

This PR adds type hints to the string-based id parameters and properties.

The resulting experience is less awesome than I hoped. Effectively, it only makes a difference in NSwag/Kiota when TId is a guid (uuid in OpenAPI). This is because client generators don't generate a numeric type when the type is string, despite a type hint such as int64. That's understandable, because it would produce an invalid JSON:API request (sending "id": 3 would fail because the value must be a string).

The type hints do show up in SwaggerUI for all TId types, which could be helpful.

A corner case surfaced in NSwag when using Guid as the id type and /GenerateNullableReferenceTypes is turned on, but GenerateOptionalPropertiesAsNullable and GenerateOptionalParameters are off. In that case, the generated type for the Id property becomes Guid (non-nullable), so omitting it from the request body sends Guid.Empty, which produces an error. This is a problem when ClientIdGenerationMode is set to Allowed, because there's no way to not send the ID. To fix that, TId must be Guid?, so a nullable type gets generated. However this is such a corner case that I've updated the existing tests to run with GenerateOptionalPropertiesAsNullable and GenerateOptionalParameters turned on, which I expect users to do anyway.

Once we update to NSwag v14, its MSBuild properties enable us to set the switches we need/recommend from the JsonApiDotNetCore.OpenApi.Client.NSwag NuGet package via an imported .props file, so the limitation is even less likely to surface.

Closes #1572.

QUALITY CHECKLIST

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.45%. Comparing base (92e5590) to head (a18666c).

Additional details and impacted files

@@             Coverage Diff             @@
##           openapi    #1595      +/-   ##
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+ Coverage    91.42%   91.45%   +0.02%     
===========================================
  Files          412      413       +1     
  Lines        13442    13479      +37     
  Branches      2102     2104       +2     
===========================================
+ Hits         12290    12327      +37     
  Misses         747      747              
  Partials       405      405              

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