[ROS2] Add Yaml Variables to config files

[Amal Nanavati]

YAML has a way to define variables to re-use throughout the file: https://docs.geoserver.org/main/en/user/styling/ysld/reference/variables.html

We should move any variables that are re-used across the file, e.g., staging configuration, above plate configuration, resting configuration, action types, etc. to be YAML variables to simplify the file and make it more readable.

[Amal Nanavati]

This is not yet supported in RCL: code, issue.