Allow list of groups provided by SAML Source Provider

[maxaudron]

Is your feature request related to a problem? Please describe.
When federating with a SAML source the SAML Attributes are flattened making it impossible to map all the groups using a custom policy like suggested in #6467
The offending code is here:

authentik/authentik/sources/saml/processors/response.py

Lines 216 to 218 in 12f2484

	# Flatten all lists in the dict
	for key, value in attributes.items():
	attributes[key] = BaseEvaluator.expr_flatten(value)

Describe the solution you'd like
Allow for a predefined attribute like groups to remain as a list.