erp5_credential: add assignment request
This is a new module usefull to manage the assignments, without having write permission on a person document.
When activated on the system preference, the user assignmentsAssignmentwill be replicated onAssignment Requestin the newassignment_request_module`.
This allows to have different permissions on those objects, and so, allow some user to manage the assignment requests without having write/read permissions on the persons.