Rewrite resource storage and add mimic test

[christophfroehlich]

I changed the storage of the component to be handled by the resource manager instead of inside of the component (deprecated).
The initial values of the handles are set by the RM -> removes some code here.

Mimic handling is similar to ros-controls/ros2_control#2571

I added a mimicked gripper test (similar to gz_ros2_control_demos) to verify that it still works.

I'm not sure if it works now with velocity command interface, and for example position+velocity state interface (did the original version work?): I'll have to add another test for something like this.