add support for connection migration (#4960)

interface.go

@@ -205,6 +205,8 @@ type Connection interface {
 	SendDatagram(payload []byte) error
 	// ReceiveDatagram gets a message received in a datagram, as specified in RFC 9221.
 	ReceiveDatagram(context.Context) ([]byte, error)
+
+	AddPath(*Transport) (*Path, error)
 }
 
 // An EarlyConnection is a connection that is handshaking.