enable scan_edge to retrive reverse edge

[kikimo]

GraphStorageClient.scan_edge() lack option to retrieve reverse edge despite the fact that it's supported by the underlying rpc in ScanEdgeRequest.EdgeProp.EdgeType:

    def scan_edge(self,
                  space_name,
                  edge_name,
                  prop_names=[],
                  limit=DEFAULT_LIMIT,
                  start_time=DEFAULT_START_TIME,
                  end_time=DEFAULT_END_TIME,
                  where=None,
                  only_latest_version=False,
                  enable_read_from_follower=True,
                  partial_success=False):