Make an external simulator backend

[knmcguire]

We can make an external simulator backend, which instead of starting a simulator, sends out topics by the motor outputs of the backend, the desired pose and twist messages per crazyflie. These topics will be read by a simulator with a ROS2 node, either Webots or Gazebo, and the integrated Crazyflie or any quadcopter will react accordingly.

I think usually twist messages is the standard, but since we also want to show trajectory control, it will be important for the drone to react on desired poses as well (or at least position). For this we do have to use the generic topic type pose or twist obviously.