Add rtp input/output sdp file support

[a-wing]

Look like ffmpeg out put SDP:

Display SDP content:

SDP:
v=0
o=- 0 0 IN IP4 127.0.0.1
s=No Name
c=IN IP4 127.0.0.1
t=0 0
a=tool:libavformat 61.1.100
m=video 5004 RTP/AVP 96
b=AS:256
a=rtpmap:96 VP8/90000

Or use flag -sdp_file

ffmpeg -re -f lavfi -i testsrc=size=640x480:rate=30 -vcodec libvpx -f rtp 'rtp://127.0.0.1:5004' -sdp_file stream.sdp

---

Input SDP

Look like ffplay:

ffplay -protocol_whitelist rtp,file,udp -i stream.sdp

[a-wing]

whipinto input sdp in 78861054019a48dfc39b434193deb628a7660589