Add MPEG4 format for RTSP on sink

Hi,
We are able to set up RTSP server on Xavier through test-launch:

$ ./test-launch "videotestsrc ! avenc_mpeg4 ! rtpmp4vpay name=pay0 pt=96"

Add decode it through VLC player on host PC. Please add avenc_mpeg4 to create_udpsink_bin() for a try.

FYR, there is guidance of running RTSP streaming in Jetson Nano FAQ