Fix gripper mimic joint control

[fabid]

What does this implement/fix?

Fix for the broken mimic joint where only 1 of the 2 gripper finger would move.

The support for mimic has been fixed for some time with this PR ros-controls/gazebo_ros2_control#107 and it shows that the mimic joint specification was missing from the ros2_control specification.
The first commit adds the specification and allows the gripper to be controlled properly

However, while the gripper works with moveit2 after this first commit, a bunch of error messages are generated at runtime:
[move_group-3] [ERROR] [1692102600.709104192] [moveit_robot_model.robot_model]: Joint 'crane_x7_gripper_finger_b_joint_mimic' not found in model 'crane_x7'

The second commit adds a dummy joint to prevent this. This is only a workaround (suggested here: ros-controls/gazebo_ros2_control#173.

There might be some more elegant solution and possible resolution is still discussed

ROBOTIS-GIT/turtlebot3_manipulation#66
ros-controls/gz_ros2_control#96

Does this close any currently open issues?

Fixes rt-net/crane_x7_ros#161

How has this been tested?

By running ros2 launch crane_x7_gazebo crane_x7_with_table.launch.py and planning arm/gripper control from RViz/Moveit

Any other comments?

The automatic test suite via github workflow seems to fail on our fork, however it looks unrelated to the change.

Checklists

• I have read the CONTRIBUTING guidelines.
  • If there is no guideline in the repository, rt-net organization's guideline applies.
• I have checked to ensure there aren't other open Pull Requests for the same change.

[Kuwamai]

Thanks! We will check next week.

[ShotaAk]

Thanks! The gripper works fine without move_group errors.

After the mimic joint problem is solved, we would remove the dummy link and the mimic joint. 👍

mimic_joint-2023-08-24_14.49.51.mp4