SFU: Register the ssrc-audio-level RTP extension (#373)

All major browser negotiate this. Adding it to ion-sfu means it will
get passed on to ion-avp, which can then do active speaker detection.

I noticed TransportCCURI was video only, and this one is audio only, so
I split the registration into two loops.

Updates pion/sdp dependency to 3.0.4

go.mod

@@ -15,7 +15,7 @@ require (
 	github.com/pion/ion-log v1.0.0
 	github.com/pion/rtcp v1.2.6
 	github.com/pion/rtp v1.6.2
-	github.com/pion/sdp/v3 v3.0.3
+	github.com/pion/sdp/v3 v3.0.4
 	github.com/pion/transport v0.12.2
 	github.com/pion/turn/v2 v2.0.5
 	github.com/pion/webrtc/v3 v3.0.3

go.sum

@@ -309,6 +309,8 @@ github.com/pion/sctp v1.7.11 h1:UCnj7MsobLKLuP/Hh+JMiI/6W5Bs/VF45lWKgHFjSIE=
 github.com/pion/sctp v1.7.11/go.mod h1:EhpTUQu1/lcK3xI+eriS6/96fWetHGCvBi9MSsnaBN0=
 github.com/pion/sdp/v3 v3.0.3 h1:gJK9hk+JFD2NGIM1nXmqNCq1DkVaIZ9dlA3u3otnkaw=
 github.com/pion/sdp/v3 v3.0.3/go.mod h1:bNiSknmJE0HYBprTHXKPQ3+JjacTv5uap92ueJZKsRk=
+github.com/pion/sdp/v3 v3.0.4 h1:2Kf+dgrzJflNCSw3TV5v2VLeI0s/qkzy2r5jlR0wzf8=
+github.com/pion/sdp/v3 v3.0.4/go.mod h1:bNiSknmJE0HYBprTHXKPQ3+JjacTv5uap92ueJZKsRk=
 github.com/pion/srtp/v2 v2.0.1 h1:kgfh65ob3EcnFYA4kUBvU/menCp9u7qaJLXwWgpobzs=
 github.com/pion/srtp/v2 v2.0.1/go.mod h1:c8NWHhhkFf/drmHTAblkdu8++lsISEBBdAuiyxgqIsE=
 github.com/pion/stun v0.3.5 h1:uLUCBCkQby4S1cf6CGuR9QrVOKcvUwFeemaC865QHDg=

pkg/sfu/mediaengine.go

@@ -69,9 +69,12 @@ func getPublisherMediaEngine() (*webrtc.MediaEngine, error) {
 		if err := me.RegisterHeaderExtension(webrtc.RTPHeaderExtensionCapability{URI: extension}, webrtc.RTPCodecTypeVideo); err != nil {
 			return nil, err
 		}
-		if extension == sdp.TransportCCURI {
-			continue
-		}
+	}
+	for _, extension := range []string{
+		sdp.SDESMidURI,
+		sdp.SDESRTPStreamIDURI,
+		sdp.AudioLevelURI,
+	} {
 		if err := me.RegisterHeaderExtension(webrtc.RTPHeaderExtensionCapability{URI: extension}, webrtc.RTPCodecTypeAudio); err != nil {
 			return nil, err
 		}