allow control of group membership info returned by UsersQuery

The results of UsersQuery include group memberships for all returned users. The server API includes a query flag that controls whether this membership information includes only direct memberships, or also includes indirect memberships in product configurations that contain user groups that contain the user. But umapi-client was not exposing control over this flag. This release exposes the server's directOnly query parameter as direct_only boolean flag to the UsersQuery initializer. The default value of the flag is True, meaning only include direct memberships in the returned data.