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.