code: def fetch(self):
          
          params = values.of({})
  
          payload = self._version.fetch(
              'GET',
              self._uri,
              params=params,
          )
  
          return AvailableAddOnExtensionInstance(
              self._version,
              payload,
              available_add_on_sid=self._solution['available_add_on_sid'],
              sid=self._solution['sid'],
          )
docs: Fetch a AvailableAddOnExtensionInstance
  
          :returns: Fetched AvailableAddOnExtensionInstance
          :rtype: twilio.rest.preview.marketplace.available_add_on.available_add_on_extension.AvailableAddOnExtensionInstance