TUN-6617: Dont fallback to http2 if QUIC conn was successful.

cloudflared falls back aggressively to HTTP/2 protocol if a connection
attempt with QUIC failed. This was done to ensure that machines with UDP
egress disabled did not stop clients from connecting to the cloudlfare
edge. This PR improves on that experience by having cloudflared remember
if a QUIC connection was successful which implies UDP egress works. In
this case, cloudflared does not fallback to HTTP/2 and keeps trying to
connect to the edge with QUIC.

connection/event.go

@@ -5,6 +5,7 @@ type Event struct {
 	Index     uint8
 	EventType Status
 	Location  string
+	Protocol  Protocol
 	URL       string
 }