docs: def application_properties(self, key=None):
          """Return the mutable server application properties.
  
          :param key: the single property to return a value for
          :type key: Optional[str]
          :rtype: Union[Dict[str, str], List[Dict[str, str]]]
  
          """
          params = {}
          if key is not None:
              params['key'] = key
          return self._get_json('application-properties', params=params)