RTP/SDP for Opus Multistream

This document specifies RTP/SDP signaling for Opus multistream (multi-channel) operation, enabling negotiation of layouts such as 5.1 and 7.1 in real-time communications. It defines an SDP encoding name and format parameters to describe multistream configurations, and specifies Offer/Answer procedures for interoperable negotiation. This document does not define new Opus codec behavior. It extends the the SDP signaling defined in [RFC7587] and reuses the channel-mapping semantics defined in [RFC7845].