Enhancement Add PROTOSpawner

[ygoumaz]

Resolves #818.

• Remove spawn_urdf_robot service and its callback.
• Remove URDFSpawner node.
• Remove webots_ros2_importer module.
• Refactor spawn_node_from_string callback to spawn_proto_robot callback (drafted in this PR):
  • The node string is now in request.robot.robot_string.
  • URLs of PROTOs are in request.robot.proto_paths and must be added to IMPORTABLE EXTERNPROTO (Add new elements to IMPORTABLE list from Supervisor API webots#6316).
  • Local PROTO URLs must be converted in function of the platform (WSL, macOS, Linux).
  • Local assets (textures, etc.) URLs contained in the node string should also be converted.
  • On macOS, local assets should be copied to the shared folder.
• Add PROTOSpawner node (drafted in this PR).
• Update webots_ros2_universal_robot to spawn UR5e from PROTO.
• Official ROS 2 documentation:
  • Update Ros2Supervisor tutorial to the new service architecture.
  • Create new tutorial on how to spawn a PROTO using the launch file and the PROTOSpawner or directly via the service.