Add is_connected property to NetworkBase class

NetworkBase.is_connected calls _wifi.is_connected which calls esp.is_connected

adafruit_portalbase/network.py

@@ -686,3 +686,8 @@ class NetworkBase:
 
             values = json.dumps(json_data)
         return values
+
+    @property
+    def is_connected(self):
+        """Return whether we are connected."""
+        return self._wifi.is_connected