Multiple RTX in RTCRtpCapabilities.codecs

[aboba]

To allow for capabilities negotiation, it makes sense to have multiple RTX codecs advertised in capabilities, one for each codec that is being retransmitted. Although RTX could be understood to apply to all codecs of a given kind, having the preferred payload type enables capability-exchange signaling. The spec already says this, but could use more explanation.

[robin-raymond]

This makes sense to me with the apt in the capability.