Use member classes for service and action child interfaces

[jacobperron]

In short, I think it is more intuitive for users to reference Request, Response, Goal, Feedback, and Result interfaces as members of the top-level interface instead of having them as siblings of the top-level interface.

For example,

examples_interfaces.srv.AddTwoInts.Request

versus how they are currently referenced

examples_interfaces.srv.AddTwoInts_Request

Accessing these classes as members is also similar to how it's done in C++ and Python.

[esteve]

@jacobperron makes total sense. I tried getting that working, but I've forgotten why I couldn't. In any case, it'd be great to use .Request / .Response instead.