SP AuthnRequest should add authncontextclassref that should be configurable

Add configuration to the handler on the sp side that generates the authnrequest to add an authncontextclassref such as:

urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport

Maybe can use shorthands such as password,kerberos,x509 etc for the standard class refs. If not standard shorthand, we use the fqn.