Add feature for constants exercise

[mikedamay]

Task 5 of the constants exercise is to "Ensure that the developers cannot be tampered with". The intention here is to encourage the student to wrap the developers dictionary in an instance of ReadOnlyDictionary<string, Identity> in the Authenticator.GetDevelopers() method and to return that type or IReadOnlyDictionary<string, Identity> from the method.

This concept is expressed elsewhere as "preventing code circumventing the published or formal API of a class".