ActuatorStateInterface to be aggregator of multiple smaller interfaces?

[bmagyar]

Fully depicting what I mean by a UML:

This'd allow for one to actually implement the actuator state interface they want in their RobotHW while clients could specify only the interfaces they require from an actuator handle.

[jordan-palacios]

Great representation.

This is how I exactly how I envisioned the new ActuatorStateInterface (see #1). My concern is how we couple this with the JointStateInterface and Transmission.