multiple lidar support

[Jakub-Krakowiak]

1. implemented gazebo system (server) plugin interface - Configure, PreUpdate and PostUpdate functions
2. connected RGL using FetchContent
3. added mesh detection from gazebo internals (both primitive and external meshes)
4. added the meshes to rgl scene
5. implemented rgl lidar entity in gazebo
6. pose updates from gazebo to rgl for both lidar and other entities
7. rgl ray tracing hits visible in GUI if RGLGuiPlugin is loaded
8. divided RGLGazeboPlugin into three parts: RGLServerPluginManager, RGLServerPluginInstance and RGLGuiPlugin (since the visualization does not need rgl)
9. RGLServerPluginManager manages adding meshes and entities to the rgl scene, while RGLServerPluginInstance takes care of raycasting and publishing PointCloudPacked messages. You can create as many RGLServerPluginInstances as you like - there is multiple lidar support
10. added raycasting while simulation is paused

[prybicki]

Part 1 of the review: RGLServerPluginManager and its dependencies.

• Most of the code looks good, especially RGLServerPluginManager.cc is very clean and readable.
• Some parts would benefit from a refactor, e.g. the code that deals with mesh & entity handling.
• Please consider using some using directives in .cc files to make types shorter.
• Please make sure errors are well reported.
• Function names could be more precise to a new reader.

[prybicki]

Left some final thoughts; to be addressed on v11-support branch. This one can be merged to main.