Support Java codegen for the OpenAPI backend
Initial implementation for a codegen feature in the OpenAPI backend. This is based on the OpenAPI Generator tool, which supports many different languages.
At this point, only the Java sources are generated with no support to detect the runtime dependencies, which will be added as this PR progresses.
Exciting! I was actually planning to do the same thing but for Python when I've got some spare time, but now I'll definitely hold that off and piggback off of the work here once it's merged. 🎉
@jyggen sounds like a plan, we can defo work together on adding support for more languages incrementally. Also, feel free to provide with any feedback/ideas where this implementation may differ from what you had in mind.