Create a simulated dispenser adapter that works with MoveIt!

[mxgrey]

To fill in some functionality of the Ionic demo we need a dispenser adapter that can issue requests to MoveIt! to generate motion plans and drive the ros2_control controllers. Currently we have a "teleportation" dispenser, but this is not as compelling of a functionality demo as we would like.

Ideally we'd have something like this:

• A simple rclpy node that listens for DispenserRequest
• For simplicity the node is configured to just grab items at a preset pose
• When a request comes in, the node should forward the request to a MoveIt! action to generate and execute a plan, using the feedback to identify when the request is finished
• Multiple simultaneous requests should be queued up and executed one at a time