Closing launch file if required composable node shuts down

[ksuszka]

Feature request

Feature description

ROS1 launch system has "required" parameter. In ROS2 launch system the same functionality can be implemented using on_exit event. However this only works for standalone executables. It would be nice to have the same functionality for composable nodes.

Implementation considerations

At the moment we implemented ugly workaround. We periodically check list of nodes in a composable container and shutdown the launch process if required nodes are missing.