SDP grouping attribute (a=group:) for related media sections.
What problem this solves
WebRTC BUNDLE groups several m= lines onto one ICE/DTLS transport via
a=group:BUNDLE. SFUs and browsers read the semantics and ordered mid list
to know which sections share a socket.
BUNDLE groups multiple m= lines onto one transport (common in WebRTC so
audio, video, and application share one ICE/DTLS association).
Wire form: a=group:<semantics> <mid> ...