EXPERIMENTAL: Add effort command interface to hardware interface

[urfeex]

This is just a proof of concept to make the robot work. This still requires some work.

Requires UniversalRobots/Universal_Robots_ROS2_Description#302 and https://github.com/UniversalRobots/Universal_Robots_Client_Library/tree/torque_control to work, hence the draft.

ToDo's:

• Implementation
• Test on a real robot
• Test in simulation (torque control doesn't work with URSim)
• Write tests (torque control doesn't work with URSim -- we cannot test much)
• Write documentation