Allow complex lookups for User

[paulhfischer]

I was wondering if you would be open for a possibility to allow complex lookups for the user model.
My proposal would be to allow another setting (e.g. CUSTOM_LOOKUP which is passed to get_or_create_user and would replace user_lookup_key and user_lookup_value if set.

My use case is that i have an ID field which isn't necessary defined and i want to provide a "fallback" field by using a query like Q(my_id="uid") | Q(email)="mail".

[peppelinux]

I'd consider only to move this

djangosaml2/djangosaml2/backends.py

Line 271 in 8bd4bf8

user_query_args = {

in a method, like "get_user_query_args" and let developers inherits this and do their own