Add support for Microsoft.AspNetCore.OpenApi

[einari]

As we have with the Swagger/Swashbuckle support, we should have a similar support for the Microsoft.AspNetCore.OpenApi way of generating OpenAPI docs.

This involves modifications of the schema for:

• ConceptAs<> types - make every model / property / parameter recursively use the underlying value type and not be represented as a complex type
• Add all HTTP status codes that our action filters uses with correct mime type of application/json
• Enums - supporting names, even though they're integers
• Support [FromRequest] objects