Add requested authn context for backend requests
Add requested authn context for backend requests

Use acr_mapping from the backend config to generate a
RequestedAuthnContext node in an AuthnRequest.

acr_mapping configuration is extended so that each entry is a dictionary
with two possible keys:
- class_ref: defines the content of the AuthnContextClassRef node
- comparison: defines the Comparison attribute

its-dirg/SATOSA,SUNET/SATOSA,irtnog/SATOSA,irtnog/SATOSA,SUNET/SATOSA