Introduce relative topic names for better multi-robot support #3420
Description
Desired behavior
When using the same SDF to spawn multiple robot instances, topics should be automatically isolated to avoid conflicts.
Currently, if topics are explicitly defined (e.g., cmd_vel), all robots share the same topic, causing interference.
Alternatives considered
Some plugins (e.g., AckermannSteering) provide a subtopic that prefixes topics with the model name.
However, this approach is not consistent across plugins and introduces extra parameters.
Implementation suggestion
Introduce support for relative topic names:
- Topics starting with
/→ treated as absolute - Otherwise → treated as relative and automatically prefixed with the model name
This provides a unified behavior without introducing additional parameters.