Allow generating an interface class for specific Java classes

[stuartmorgan]

For unit testing code that's using FFI calls, we commonly want to mock/stub out at least some of the classes we are calling. In the Pigeon world we just mock the generated API, but a generated JObject subclass appears to have about 150 methods that aren't related to the specific object.

It would be nice in the configuration to be able to indicate that for specific classes I want simple interfaces of just the actual Java methods, for mocking purposes.