Gazebo: Add WildThumper with Lidar

[srmainwaring]

Add a variant of the wild thumper rover with a RP A1 Lidar, reusing the mesh files from the existing wildthumper model.

Comments

Ideally we would use model nesting to compose the lidar link with the existing willdthumper model. Unfortunately because of limitations in the sdformat_urdf plugin (see: ros/sdformat_urdf#20) Gazebo and ROS cannot handle different resource prefixes consistently. There is a work-around that allows us to display the SDFormat robot model in both Gazebo and rviz provided the model is not nested. The required prefix is substituted at launch time in the ROS 2 Python launch file. The approach requires that all resources are specified a single SDF file.

When the upstream issues are addressed the lidar variant of the wild thumper model can be simplified.

See also

• Rover: add rover example ardupilot_gz#33
• Rover: implement external control ardupilot#25204