[FOR DISCUSSION ONLY] XD-647: Http message unmarshalling

Pending cleanup as well as integration tests for the http source module. Opening a PR for easy discussion.

My thoughts:
- This is not http module specific, this is not even *http* specific actually, could benefit any XD/SI component that manipulates an InputStream with some knowledge of a Content-Type
- Given that, the classes could be moved to other project(s)
- The "HttpMessageConverterSupport" class may end up implementing HttpMessageConverter itself and be GoF composite (not sure whether getSupportedMediaTypes() would return the union or the intersection though). This behavior should even be in Spring core though, as it's seen over and over again (maybe with split the default converters registration elsewhere)
- Not happy with "unmarshallTo" name at all
- whatever its name, the combination with --outputType feels weird.