Expose TypeHash and TypeDescription on generated types

[emersonknapp]

Feature request

Feature description

The C generated code for types exposes this info, we need to bridge it through the generated python types.

An initial aborted attempt was at #191
Related to ros2/ros2#1159

Implementation considerations

Access to the type_description_interfaces types at this stage. There is rosidl_runtime_c versions - maybe we move some implementation up to rclpy where the message types will be available?