Controller Spawner launch extension

[SuperJappie08]

Is your feature request related to a problem? Please describe.
I was attempting to do some dynamic controller configuration and remapping from a launch file (using the xml front-end).
During this I ran into parsing issues with the dynamically created --controller-ros-args, since launch attempts to split the arguments in an unescapable way (hardcoded it works fine, so no real issue in a more normal case).
Furthermore, it is currently not easily possible to set controller parameters that are defined only in a launch file.

Describe the solution you'd like
A launch extension, like provided with ros_gz_sim and ros_gz_bridge in which special launch actions get exposed to spawn and unspawn hardware.
This could be part of the controller_manager package (similar to the Gazebo case) or a separate package (launch_ros2_control), similar to launch_ros.

Describe alternatives you've considered
Writing a Python launch file, which is inconvenient since it is introducing a lot more more complexity to do something relatively simple.
While using a launch front-end (like launch_xml or launch_yaml) could also easily suffice.

Additional context
I would be willing to help realize this feature.

[SuperJappie08]

I started experimenting with this idea, and this could be the concept for the spawner.

Python:

ControllerSpawner(
    controllers=[
        Controller(
            name="some_controller",
            remaps=[("from", "to")],  # These get converted to `--controller-ros-args -r <controller-name>:from:=to`,
            parameters=[...],  # Parameters specific for this controller, gets put in generated file, like how launch_ros does it
        ),
    ],
    # Optional other args,
   spawner_name='...',
   controller_manager=...,
   spawner_remaps=...,  # remaps for the spawner itself 
   controller_parameters=..., # Global parameters attached to all controllers
)

XML

<controller_spawner> <!-- A lot of the optional attributes, like name go here -->
  <controller name="<CONTROLLER_NAME>">
    <remap from="from" to="to"/>
    ....
    <param .../> <!-- accepts all the launch_ros Node Param forms, but specific for this controller -->
  </controller>
  <controller_param ... /> <!-- accepts all the launch_ros Node Param forms, but specific for all spawned controllers -->
  <spawner_remap .../> <!-- Remaps for the spawner -->
</controller_spawner>

It would probably also be useful to support conditions on the controllers.

[SuperJappie08]

I made a working version for Jazzy as a separate package called launch_ros2_control.
It currently only supports the most important features of the controller spawner.
(The main thing missing is remapping and parameters on the spawner itself).

I will add some more docs to it today.

[github-actions]

This issue is being labeled as stale because it has been open 45 days with no activity. It will be automatically closed after another 45 days without follow-ups.

[SuperJappie08]

(nudge to keep alive, in case somebody is interested)