TUN-4597: Add a QUIC server skeleton

- Added a QUIC server to accept streams
- Unit test for this server also tests ALPN
- Temporary echo capability for HTTP ConnectionType

vendor/github.com/lucas-clemente/quic-go/internal/ackhandler/send_mode.go

@@ -0,0 +1,40 @@
+package ackhandler
+
+import "fmt"
+
+// The SendMode says what kind of packets can be sent.
+type SendMode uint8
+
+const (
+	// SendNone means that no packets should be sent
+	SendNone SendMode = iota
+	// SendAck means an ACK-only packet should be sent
+	SendAck
+	// SendPTOInitial means that an Initial probe packet should be sent
+	SendPTOInitial
+	// SendPTOHandshake means that a Handshake probe packet should be sent
+	SendPTOHandshake
+	// SendPTOAppData means that an Application data probe packet should be sent
+	SendPTOAppData
+	// SendAny means that any packet should be sent
+	SendAny
+)
+
+func (s SendMode) String() string {
+	switch s {
+	case SendNone:
+		return "none"
+	case SendAck:
+		return "ack"
+	case SendPTOInitial:
+		return "pto (Initial)"
+	case SendPTOHandshake:
+		return "pto (Handshake)"
+	case SendPTOAppData:
+		return "pto (Application Data)"
+	case SendAny:
+		return "any"
+	default:
+		return fmt.Sprintf("invalid send mode: %d", s)
+	}
+}