[launch] add new concept for LaunchArgument

[wjwwood]

The idea behind the concept of LaunchArgument is that it should be something you can declare (perhaps as an Action) in a launch description which indicates some input configurations to the launch description.

If the launch description is included by another launch description then the arguments would be part of that action, but if the launch description is "run" by ros2 launch it would be possible to pass them as command line arguments.

This is partially equivalent to the <arg> tag in roslaunch from ROS 1:

https://wiki.ros.org/roslaunch/XML/arg

It should take a name, optionally a default value, optionally documentation for the argument, and possibly other things commonly associated with command line arguments, like being a flag rather than a key-value option or with special validation logic (is it the right type or in the right range)? Initially it should always have a type of string, and it can be evaluated by the things consuming it.

The result (default value or user provided) would be stored in a LaunchConfiguration of the same name.