Improve: UDP relay refactor (#441)

Co-authored-by: Dreamacro <Dreamacro@vip.qq.com>

constant/adapters.go

@@ -109,3 +109,21 @@ func (at AdapterType) String() string {
 		return "Unknown"
 	}
 }
+
+// UDPPacket contains the data of UDP packet, and offers control/info of UDP packet's source
+type UDPPacket interface {
+	// Data get the payload of UDP Packet
+	Data() []byte
+
+	// WriteBack writes the payload with source IP/Port equals addr
+	// - variable source IP/Port is important to STUN
+	// - if addr is not provided, WriteBack will wirte out UDP packet with SourceIP/Prot equals to origional Target,
+	//   this is important when using Fake-IP.
+	WriteBack(b []byte, addr net.Addr) (n int, err error)
+
+	// Close closes the underlaying connection.
+	Close() error
+
+	// LocalAddr returns the source IP/Port of packet
+	LocalAddr() net.Addr
+}