Enable reading Launch Configuration from yaml file

[tonynajjar]

Feature request

Feature description

As far as I know, the only way to pass LaunchConfigurations to a launch file is through the command line e.g. ros2 launch nav2_bringup tb3_simulation_launch.py headless:=False. It would be useful to be able to load LaunchConfiguration via a YAML file e.g. ros2 launch nav2_bringup tb3_simulation_launch.py --launch-arguments /path/to/yaml.

This is useful when you want to version control the launch configuration for configuration management purposes (e.g. in environment X, we don't want to run a certain monitoring node)

Implementation considerations

Some requirements from my side:

• specify multiple yaml files with the same overriding concepts as for nodes
• launch configurations should be namespaced by e.g. the launch file name for them to be loaded (same as for node).

[tonynajjar]

Here is a very basic working example (without all the requirements): https://github.com/angsa-robotics/launch_ros/commit/8cbb490b4442b22bbe81d0abea86f0821527112b