Fix final type mismatches between front and back ends

Fix #170 (closed)

Following specs at https://ewokscore.readthedocs.io/en/latest/definitions.html#graph-attributes and https://gitlab.esrf.fr/workflow/ewoks/ewoksserver/-/blob/9e411d193626ff9c1f4672fc08144b52896b64fa/src/ewoksserver/resources/api.py