Spruce up OpenApiSchema generation

[captainsafia]

Part of #41246

We're pursuing more fully-fledged schema generation support via microsoft/OpenAPI.NET#836 but this fixes up a few things with the current logic:

• Supports more of the primitive .NET types
• Sets the Nullable attribute on types using NullabilityContext info
• Sets the format for primitive types where it is supported

[BrennanConroy]

Generally looks good. Do we need to add a couple test cases for Dictionary, Time* types, and IEnumerable?

The PR description says it closes the issue, the issue talks about handling TryParse and default values. Does that still need to be tracked?

[captainsafia]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[captainsafia]

The PR description says it closes the issue,

I changed the issue text here. There's actually potentially other things that will happen in this schema generation area.

The PR description says it closes the issue, the issue talks about handling TryParse and default values. Does that still need to be tracked?

Their omission is intentional here and mainly has to do with the fact that Stuff's Complicated ™️ .

• Handling TryParse requires that we be able to create a schema for a potentially complex object, such as a TryParse that targets a record Location(int latitude, int longitude). In that case, we need to be able to construct a schema for an object containing latitude and longitude properties. There's a catch here, however, which is that we still want to be allow users to treat this types as string parameters to their request (so the interactive experience in Swagger UI) works.
• The Default in the OpenApiSchema must implement the OpenApiAny type which means that any default value you pass to it has to either implement OpenApiAny or map to it, which requires being able to generate a schema for the type.

Edit: I'm also noodling on an alternative approach for this problem which involves "merging" with the Swashbuckle-generated schema instead of overriding it altogether (unless the user explicitly did so in their WithOpenApi invocation).