fix(netx): make sure we save quic udp conn events (#423)

https://github.com/ooni/probe-cli/pull/421 was wrong because we need
a more rich interface for quic-go to call ReadMsgUDP.

With this commit, we use such an interface: OOBCapablePacketConn.

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

internal/quicx/quicx.go

@@ -1,13 +0,0 @@
-// 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)
-}