Support for Kiota to generate OpenAPI clients

[bkoelman]

This PR adds a support library, tests, examples and documentation for using JsonApiDotNetCore with Kiota, which is a fairly new OpenAPI client generator that supports the following languages: C#, Go, Java, PHP, Python, Ruby, Swift and TypeScript.

Because the project is still young, it has some rough edges and bugs, for which we provide workarounds where feasible:

• Everything is always nullable [Multiple languages] properties are generated as nullable/optional, despite required/nullable usage in OAS schema microsoft/kiota#3911
• Query strings are inaccessible How to pass a free-format query string? microsoft/kiota#3800
• Incompatible classes are generated for includes Multiple identical classes are generated for a component schema with allOf inheritance microsoft/kiota#4191
• HTTP 304 (Not Modified) is not properly recognized Kiota throws ApiException when server returns 304 microsoft/kiota#4190
• Warning for format:uri in HTTP Location header Kiota warns that format:uri is not supported in headers microsoft/kiota#4227
• Unstable content of kiota-lock.json file kiota-lock.json changes depending on OS microsoft/kiota#4228
• Stream response for HEAD request Stream response for HEAD endpoint microsoft/kiota#4245
• Poor exception message Take exception message from OpenAPI document microsoft/kiota#4349

In preparation, this PR contains various refactorings. See the individual commit messages for details.

Changes that affect usage:

• Renamed package JsonApiDotNetCore.OpenApi.Client to JsonApiDotNetCore.OpenApi.Client.NSwag.
• New package JsonApiDotNetCore.OpenApi.Client.Kiota.
• The IMvcCoreBuilder mvcBuilder parameter has been removed from AddOpenApi().

Changes that affect internal development:

• Renamed projects/namespaces to distinguish between NSwag and Kiota.
• When running end-to-end tests, the incoming and outgoing HTTP traffic is logged to the test output window.
• NSwag-generated client code is now under source control, so we'll know when something changes.
  Reverted that, due to https://youtrack.jetbrains.com/issue/RSRP-496113/Newtonsoft.Json-conflict-in-CleanupCode.
• The generated swagger.g.json files in tests now live in the server test project; client tests point to it.

Closes #1458.

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 90.76%. Comparing base (54c401c) to head (fccd8dd).

Additional details and impacted files

@@           Coverage Diff            @@
##           openapi    #1479   +/-   ##
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  Coverage    90.75%   90.76%           
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  Files          390      391    +1     
  Lines        12883    12896   +13     
  Branches      2044     2046    +2     
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+ Hits         11692    11705   +13     
  Misses         776      776           
  Partials       415      415           

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[bkoelman]

@verdie-g Would you like to review this PR?

[verdie-g]

I can have a look if there is no hurry to merge it.

[bkoelman]

I can have a look if there is no hurry to merge it.

There's no urgency. Also, the changes are relevant to be aware of for future development. Probably easiest to review per commit.