Fix issue #4570 by supporting optional msid attribute in WebRTC SDP negotiation, enabling compatibility with libdatachannel and other clients that don't include msid information.

SRS failed to negotiate WebRTC connections from libdatachannel clients because:

• libdatachannel SDP lacks a=ssrc:XX msid:stream_id track_id attributes
• SRS required msid information to create track descriptions
• According to RFC 8830, the msid attribute and its appdata (track_id) are optional

If diligently look at the SDP generated by libdatachannel:

a=ssrc:42 cname:video-send
a=ssrc:43 cname:audio-send

It's deliberately missing the a=ssrc:XX msid:stream_id track_id line, comparing that with this one:

a=ssrc:42 cname:video-send
a=ssrc:42 msid:stream_id video_track_id
a=ssrc:43 cname:audio-send  
a=ssrc:43 msid:stream_id audio_track_id

In such a situation, to keep compatible with libdatachannel, if no msid line in sdp, SRS comprehensively and consistently uses:

• app/stream as stream_id, such as live/livestream
• type=video|audio, cname, and ssrc as track_id, such as track-video-video-send-43