After-action for :relate-style update actions on Diffo Instance resources.
Applies the :relationships argument via Relationship.relate_instance,
then reloads the instance through its primary read so preparations re-run.
Use it on any instance update action that carries
argument :relationships, {:array, :struct}:
update :relate do
argument :relationships, {:array, :struct}
change DiffoExample.Changes.Relate
end