Support multicast streams

[curtishall]

Currently we support unicast RTSP streams. A customer requires the ability for the server to receive multicast RTSP streams to lower their network usage as they have over 1000 streams on their network.

https://community.boschsecurity.com/t5/Security-Video/How-to-use-RTSP-and-multicast-streaming/ta-p/3171
https://developer.ridgerun.com/wiki/index.php/GstRtspSink_-_Multicast
https://ffmpeg.org/ffmpeg-protocols.html