Add PushRosNamespace action

[ivanpauno]

Based on discussion ros2/design#207 (review), ros2/design#207 (comment), and inspired on Node action documentation (of a feature that was documented but not implemented).

This adds a way of scoping ROS entities in a namespace.

Example usage:

LaunchDescription([
    GroupAction([
        PushRosNamespace('my_ns'),
        IncludeLaunchDescription(PythonLaunchDescriptionSource('another_launch_file'),
    ])
])

I think we should consider in the future:

• Add GroupAction and IncludeLaunchDescription alternatives in launch_ros. Add to them an extra ros_namespace argument.
• Add scoped argument to IncludeLaunchDescription action (in launch), default to True (which will break api, but I think is more "normal").

[wjwwood]

lgtm

[wjwwood]

Add GroupAction and IncludeLaunchDescription alternatives in launch_ros. Add to them an extra ros_namespace argument.

That is fine by me, though we have to address the namespacing in XML or instead of having new versions that "overload", find a way to extend the actions that are already in launch via launch_ros.

Add scoped argument to IncludeLaunchDescription action (in launch), default to True (which will break api, but I think is more "normal").

I think it would be ok to do this at the XML level, but I would not do this at the action level. I intentionally left scoping to groups as to not complicate every action that might benefit from scoping, like the include action and the set launch configuration action.

[wjwwood]

I would also not change the default behavior of scoping in the Python API, but again, I think that could be appropriate in the XML syntactic sugar.

[ivanpauno]

CI (up to test_launch_ros, launch_ros):

• Linux
• Linux-aarch64
• macOS
• Windows

[ivanpauno]

Double checking if linter is happy after 5b64bd7:

• Linux