expose GSO usage through ConnectionState (#4083)

connection.go

@@ -673,6 +673,7 @@ func (s *connection) ConnectionState() ConnectionState {
 	cs := s.cryptoStreamHandler.ConnectionState()
 	s.connState.TLS = cs.ConnectionState
 	s.connState.Used0RTT = cs.Used0RTT
+	s.connState.GSO = s.conn.capabilities().GSO
 	return s.connState
 }
 

interface.go

@@ -345,4 +345,6 @@ type ConnectionState struct {
 	Used0RTT bool
 	// Version is the QUIC version of the QUIC connection.
 	Version VersionNumber
+	// GSO says if generic segmentation offload is used
+	GSO bool
 }