Revisit built-in authentication

[alex-oleshkevich]

I want to propose the following change:

1. reduce interface of BaseUser to identity or get_identity
2. add user attribute to AuthCredentials
3. add is_authenticated to AuthCredentials

Motivation
Currently, we have to inherit user model from BaseUser. The base class adds extra attributes (display_name, is_authenticated) to the user-defined model, which may be unwanted. Also, these attributes are not used by Starlette.
My idea is to scope the auth related utilities in the AuthCredentials class and remove it from user model.

Example:

class BaseUser:
    def get_identity() -> str: ...

class UnauthenticatedUser(BaseUser): ...

class AuthCredentials:
    def __init__(self, user: BaseUser, scopes: typing.Optional[typing.Sequence[str]] = None):
        self.user = user
        self.scopes = [] if scopes is None else list(scopes)

    @property
    def is_authenticated(self) -> bool:
        return isinstance(self.user, UnauthenticatedUser)