User identificator configureable

[Baachi]

We currently have hardcoded the uid property to identify a user, but some providers does not have a uid property so they can't be used right now.

So we should rely on the nameid format and/or provide a mapping configuration for this kind of configuration.

[neufeind]

Stumbled across this as well. Workaround might be to set a value for "uid" in Classes/Authentication/SamlAuth.php. It's expected in Classes/Service/UserCreator.php that the "uid" attribute is set.
With a Microsoft-idp for example it might be use to choose a value from $attributes['http://schemas.microsoft.com/identity/claims/displayname'].