Allow `AnonymousIdentity` for `get_identity()` utils functions

[zannkukai]

Is your feature request related to a problem? Please describe.

the

invenio-access/invenio_access/utils.py

Line 14 in 87dde7f

def get_identity(user):

method could be return a value even for AnonymousUser

Describe the solution you'd like

Check if user is an anonymous. If yes, return a flask_login.AnonymousIdentity instance.

    if not user.is_anonymous:
        return AnonymousIdentity()

I will be happy to contribute if you valid this feature.