GH-232 Allow reference to a model from OpenApiContentProperty

[f1qwase]

Hi @dzikoysk,
I faced a need to send complex data along with files in a single request.
OpenAPI specification allows it: https://swagger.io/docs/specification/v3_0/describing-request-body/multipart-requests/

Currently javalin-openapi only allows description of primitive types properties in multipart/form content using fields type and format.
This PR introduces a from property to OpenApiPropertyContent annotation. The new from property is treated the same way as in OpenApiContent (it has higher priority than type and format and registers referenced type globally).

What do you think?

[dzikoysk]

Yeah, makes sense - thanks for the PR! :) Could you quickly check if it doesn't infinitely loop when a class with self-reference is used? Like:

data class Loop(val self: Loop)

It might be a good idea to add it to the existing example (or tests), just in case 😅