Make C++ typesupport use rosidl_typesupport_interface

[sloretz]

ros2/rosidl_typesupport#114 adds rosidl_typesupport_interface implementations for actions and services. It was already implemented for messages.

Right now rosidl_typesupport_cpp is defining get_action|service|message_type_support_handle<T>() in a .cpp file. I think it should be defined in a header file generated by rosidl_generator_cpp, and it should call the rosidl_typesupport_interface function to get the handle. That makes the rosidl_typesupport_interface APIs the authoritative source with the C++ function being useful sugar on top of it.

Links

Defines rosidl_typesupport_cpp::get_action_type_support_handle() here
Defines rosidl_typesupport_cpp::get_service_type_support_handle() here
Defines rosidl_typesupport_cpp::get_message_type_support_handle() here