support webrtc-datachannel sdp exchange

[duiniuluantanqin]

Currently, SRS does not support WebRTC data channel. If a client requests to establish a data channel connection, it will cause an error in the browser.

Failed to set remote answer sdp: The order of m-lines in answer doesn't match order in offer. Reject

The SDP response from SRS before modification is as follows:

v=0
...
a=group:BUNDLE 0 1
m=audio 9 UDP/TLS/RTP/SAVPF 111
...
m=video 9 UDP/TLS/RTP/SAVPF 106 114
...

After modification:

v=0
...
a=group:BUNDLE 0 1 2
m=audio 9 UDP/TLS/RTP/SAVPF 111
...
m=video 9 UDP/TLS/RTP/SAVPF 106 114
...
m=application 9 UDP/DTLS/SCTP webrtc-datachannel
...

Doubtful: ~~Is datachannel considered as a media stream? Does it count as a track? If not, it should not be placed inside the SrsMediaDesc class, but should be separated.~~

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TRANS_BY_GPT3