Cleanup of uid-on-create attribute on resource unassignment

Suppose to have an external resource that generates ids on its side and you should set for a specific mapping the "Schema to hold values for identifiers generated upon Create by the external Identity Store" with a specific plain attribute name, i.e. "uid on create".

When that resource is unassigned  from the user, group or any object, that attribute value should be cleared to guarantee consistency in future assignments of the same resource and to cleanup all info related to the removed resource.