Add hold_joints parameter (backport #251)

[mergify]

@HPCLOL posted a use case of gazebo in ros-controls/ros2_controllers#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:

passive_rrbot.mp4

---

This is an automatic backport of pull request #251 done by [Mergify](https://mergify.com).

[mergify]

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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

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

[saikishor]

LGTM