Feature: Use the defaults guard in order to determine the UserClassModel

[antoine-eurecab]

Allows more flexibility when the project does not use the conventional auth.providers.users.model

[Slund]

We could use some flexibility here as well - specifically we have 2 user providers and it could be beneficial to choose/config which to use.

I could attempt a PR, would like know if @JeffreyWay is interested in such a feature.

Thanks for an awesome package!