Port robot_state_helper to ROS2 (backport #933)

[mergify]

This pull request includes the 'robot_state_helper' node, which has been ported from the ROS1 to the ROS2 version of the driver.

---

This is an automatic backport of pull request #933 done by Mergify.

[mergify]

Cherry-pick of d4947aa has failed:

On branch mergify/bp/humble/pr-933
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit d4947aa.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   ur_dashboard_msgs/action/SetMode.action
	new file:   ur_robot_driver/doc/controller_stopper.rst
	new file:   ur_robot_driver/doc/robot_state_helper.rst
	new file:   ur_robot_driver/include/ur_robot_driver/robot_state_helper.hpp
	modified:   ur_robot_driver/launch/ur_control.launch.py
	new file:   ur_robot_driver/src/robot_state_helper.cpp
	new file:   ur_robot_driver/src/robot_state_helper_node.cpp

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   ur_robot_driver/CMakeLists.txt
	both modified:   ur_robot_driver/doc/index.rst
	deleted by us:   ur_robot_driver/doc/usage/startup.rst

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally