MarshallingDirectives, FormFieldDirectives, and more

It may not readily show but this PR has been keeping me busy for far too long.
Due to numerous interdependencies it addresses 3 tickets at the same time: #15927, #15923 and #16190.

I'm glad to finally get this beast of my table! :)
Ah, I see now, why we have `ContentTypeRange` and not `ContentType`.
The `multipartGeneralUnmarshaller` can unmarshal all `multipart/*`content generically, because all multipart content conforms to the same framing standard, which the unmarshaller decodes.
Listing all individual `multipart` media-types is not really possible though.