Add URDF model for gripper with multiple links and joints

- Created a new URDF file for the gripper, defining its structure.
- Added links: base_frame, plate, screw, craving1, craving2, craving3.
- Defined joints for the gripper's movement, including fixed and revolute joints.
- Included mesh files for visual and collision geometry.
- Set inertial properties for each link with zero mass and inertia.

src/iiwa_bringup/launch/supported/webots_spawn.launch.py

@@ -34,7 +34,7 @@ def _spawn_setup(context, *args, **kwargs):
         parameters=[
             {
                 "robot_description": description,
-                "use_sim_time": False,
+                "use_sim_time": True,
                 "set_robot_state_publisher": False,
             },
             controller,