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/send_conn.go

@@ -0,0 +1,74 @@
+package quic
+
+import (
+	"net"
+)
+
+// A sendConn allows sending using a simple Write() on a non-connected packet conn.
+type sendConn interface {
+	Write([]byte) error
+	Close() error
+	LocalAddr() net.Addr
+	RemoteAddr() net.Addr
+}
+
+type sconn struct {
+	connection
+
+	remoteAddr net.Addr
+	info       *packetInfo
+	oob        []byte
+}
+
+var _ sendConn = &sconn{}
+
+func newSendConn(c connection, remote net.Addr, info *packetInfo) sendConn {
+	return &sconn{
+		connection: c,
+		remoteAddr: remote,
+		info:       info,
+		oob:        info.OOB(),
+	}
+}
+
+func (c *sconn) Write(p []byte) error {
+	_, err := c.WritePacket(p, c.remoteAddr, c.oob)
+	return err
+}
+
+func (c *sconn) RemoteAddr() net.Addr {
+	return c.remoteAddr
+}
+
+func (c *sconn) LocalAddr() net.Addr {
+	addr := c.connection.LocalAddr()
+	if c.info != nil {
+		if udpAddr, ok := addr.(*net.UDPAddr); ok {
+			addrCopy := *udpAddr
+			addrCopy.IP = c.info.addr
+			addr = &addrCopy
+		}
+	}
+	return addr
+}
+
+type spconn struct {
+	net.PacketConn
+
+	remoteAddr net.Addr
+}
+
+var _ sendConn = &spconn{}
+
+func newSendPconn(c net.PacketConn, remote net.Addr) sendConn {
+	return &spconn{PacketConn: c, remoteAddr: remote}
+}
+
+func (c *spconn) Write(p []byte) error {
+	_, err := c.WriteTo(p, c.remoteAddr)
+	return err
+}
+
+func (c *spconn) RemoteAddr() net.Addr {
+	return c.remoteAddr
+}