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

rtcrtpencodingparameters.go

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