Add feature for constants exercise

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` in the `Authenticator.GetDevelopers()` method and to return that type or `IReadOnlyDictionary` from the method.

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