Allow retrieval of the API definition in OpenAPI 3 form

[jongeorge1]

At present, Menes provides the ability to automatically expose an API's definition via a built in /swagger endpoint, added by calling the AddSwaggerEndpoint extension method on IOpenApiDocuments during setup.

This endpoint returns the definition in JSON form using the OpenAPI v2 spec (controlled in OpenApiDocumentJsonConverter). As we move towards using more OpenAPI v3 features, we're finding that this conversion loses a lot of useful information. As such, it would be useful to retrieve the definition as per the v3 spec.

Options:

• Add a separate /openapi endpoint for v3.
• Retain the existing /swagger endpoint, but add a query param to allow specifying the required version. Default to v2 to retain existing behaviour.

[jongeorge1]

My preference is for the first option - a new endpoint. This due to the fact that Swagger v2 was donated to the OpenAPI initiative to form the basis on which OpenApi 3 was developed - so really there is no such thing as "Swagger v3".