Add RTCRtpCodingParameters for ORTC

This is a minimal subset of ORTC, but since we don't encode/decode
ourselves all we really care about is ssrc and PayloadType

Relates to #312

rtcrtpdecodingparameters.go

@@ -0,0 +1,8 @@
+package webrtc
+
+// RTCRtpDecodingParameters provides information relating to both encoding and decoding.
+// This is a subset of the RFC since Pion WebRTC doesn't implement decoding itself
+// http://draft.ortc.org/#dom-rtcrtpdecodingparameters
+type RTCRtpDecodingParameters struct {
+	RTCRtpCodingParameters
+}