This class represents a user in an Atlas App Services application.
A user can log in to the server and, if access is granted, it is possible to synchronize the local Realm to MongoDB Atlas.
Moreover, synchronization is halted when the user is logged out. It is possible to persist a user. By retrieving a user, there is no need to log in again.
Persisting a user between sessions, the user's credentials are stored
locally on the device, and should be treated as sensitive data.