Add `hold_joints` parameter (backport #251)

[mergify[bot]]

@HPCLOL posted a use case of gazebo in https://github.com/ros-controls/ros2_controllers/issues/875: If the simulation is started with an inactive controller, the robot should move with gravity as if the joints were passive.

As discussed with #250 this is currently not possible. This PR adds an parameter to enable this behavior (again).

• I'm not sure if this should be default true or false.
• I haven't found a different solution to tell GazeboSystem this parameter parsed from the gazebo plugin without changing API. Therefore, I used a ROS parameter of the gazebo_ros2_control node to pass the information.

The result looks like this if no controller was spawned:

https://github.com/ros-controls/gazebo_ros2_control/assets/3367244/45e2f56a-7173-4cee-a30e-02713127c63b

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This is an automatic backport of pull request #251 done by Mergify.

[mergify[bot]]

Cherry-pick of 7682dced640efcd67211e085cefbca23e2d1dab5 has failed:

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Changes to be committed:
    modified:   doc/index.rst
    modified:   gazebo_ros2_control/src/gazebo_ros2_control_plugin.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   gazebo_ros2_control/src/gazebo_system.cpp

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