rtpopuspay: support libwebrtc-compatible multichannel payload
When the audio has more than 2 channels, add optional fields to output caps from which webrtcbin can generate SDP in the syntax recognized by "multiopus" codec present in libwebrtc [1].
e.g. for 5.1 audio:
a=rtpmap:96 multiopus/48000/6 a=fmtp:96 num_streams=4;coupled_streams=2;channel_mapping=0,4,1,2,3,5