Have a schema description for the ros2_control tag in the URDF

[bmagyar]

Is your feature request related to a problem? Please describe.
The format of the contents of the ros2_control tag is hard to figure out and is mostly created by copying existing demos and examples or reading the parser code.

Describe the solution you'd like
Set up a schema description file (xsd) for the ros2_control tag based on the existing demos and reading the parser code (I know, cheeky ;) ) and investigate what we could use to run a validation on the tag prior to starting to parse it.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[github-actions]

This issue is being labeled as stale because it has been open 45 days with no activity. It will be automatically closed after another 45 days without follow-ups.