Adds configuration directive for RequestedAuthnContext #806

[pandafy]

Closes #806

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TODO: Update documentation

[c00kiemon5ter]

I've updated this and the config looks like this:

    "service": {
        "sp": {
            "requested_authn_context": {
                "authn_context_class_ref": [
                    "urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport",
                    "urn:oasis:names:tc:SAML:2.0:ac:classes:TLSClient",
                ],
                "comparison": "minimum",
            }
        }
    }

Internally this is converted to a proper object and serialized as XML as part of the AuthnRequest.

/cc @pandafy @peppelinux

[peppelinux]

thank you @c00kiemon5ter
djangosaml2 is resilient on breaking changes related to this feature because it adopts a custom way to handle this behaviour.
I think that your proposal is better than before, so feel free to merge it asap, I'll do my updates in djangosaml2 by my side

@pandafy ^