Add connected() in session.py to figure out session status (#2012)

* GAE add connected() in session.py

GAE add connected() in session.py to indicate if the session is still valid and available for the graph computing work.
In some case analytical engine cannot be connected, with this new function, client side can check the session status and process this special case(try to restart runtime or close session etc).

python/graphscope/client/session.py

@@ -803,6 +803,14 @@ def _send_heartbeat(self):
                     self._disconnected = False
             time.sleep(self._heartbeat_interval_seconds)
 
+    def connected(self) -> bool:
+        """Check if the session is still connected and available.
+
+        Returns: True or False
+
+        """
+        return not self._disconnected
+
     def close(self):
         """Closes this session.