Add a public API to send RTCP packets

Relates to #63

pkg/rtcp/packet.go

@@ -0,0 +1,7 @@
+package rtcp
+
+// Packet represents an RTCP packet, a protocol used for out-of-band statistics and control information for an RTP session
+type Packet interface {
+	Marshal() ([]byte, error)
+	Unmarshal(rawPacket []byte) error
+}