Ability to define custom SAML Server class

[Amertz08]

Would be nice to be able to define a custom Server class used out of the saml2 library. We use AWS XRay as an APM and often have to override methods on classes in packages to annotate the values we want to see.

Maybe something like this.

# settings.py
SAML2_SERVER_CLASS = "path.to.MyCustomClass"

Then IdPHandlerViewMixin could use the custom one or default to the standard class.