webrtc: add netsim before nicesink
In order to simulate poor network connectivity on the sender side, we need to add a 'netsim' element right before nicesink. This cannot be easily done from the application code so adding a GST_WEBRTC_NETSIM env variable to do so.
This is a bit hacky but I think it may be useful to others so opening a MR to start the discussion about what would be the proper way to integrate this.