Options for limiting the list based on gateway type and
associations.
If GATEWAY is specified, only gateways are returned. If
NON_GATEWAY is specified, only non-gateway devices are
returned. If GATEWAY_TYPE_UNSPECIFIED is specified, all
devices are returned.
If set, returns only the gateways with which the specified
device is associated. The device ID can be numeric
(num_id) or the user-defined string (id). For example,
if 456 is specified, returns only the gateways to which
the device with num_id 456 is bound.
