refactor: cleaner way of passing a UDPConn around (#421)

* refactor: cleaner way of passing a UDPConn around

Also part of https://github.com/ooni/probe/issues/1505

* Update internal/engine/netx/quicdialer/connectionstate.go

internal/quicx/quicx.go

@@ -0,0 +1,13 @@
+// Package quicx contains definitions useful to implement QUIC.
+package quicx
+
+import "net"
+
+// UDPConn is an UDP connection used by quic.
+type UDPConn interface {
+	// PacketConn is the underlying base interface.
+	net.PacketConn
+
+	// ReadMsgUDP behaves like net.UDPConn.ReadMsgUDP.
+	ReadMsgUDP(b, oob []byte) (n, oobn, flags int, addr *net.UDPAddr, err error)
+}