This interface represents a user.
It has the some of the same fields as a Firebase User.
Note that in the default implementation, we simply take the Firebase user
and use it as a FireCMS user, so that means that even if they are not mapped
in this interface, it contains all the methods of the former, such as
|The display name of the user.|
|The email of the user.|
|The profile photo URL of the user.|
|The provider used to authenticate the user.|
|The user's unique ID, scoped to the project.|