Permit to have sensor plugins as child tag of sensor sdf element instead of child of model elements

[traversaro]

One big difference between gz-sim-yarp-plugins and gazebo-yarp-plugins is that in gz-sim-yarp-plugins at the moment the sensor plugins can't be child of sensor elements. I do not remember of the details of why we did this, but I remember that we had problems when we tried to make the sensor plugin a child of sensors.

However, looking in gz-sim it seems that there are indeed some plugins that are child of sensors, see https://github.com/gazebosim/gz-sim/blob/5944188294511a60b829f88851edeaded0b72a84/examples/worlds/camera_video_record_dbl_pendulum.sdf#L312 . So perhaps this changed or we did something wrong back in time, so it would be interesting to modify the code to permit sensor plugins to be child of sensor elements in sdf.