feat(jans-auth-server): Support OAuth 2.0 Step-up Authentication Challenge Protocol (draft)

[nynymike]

OAuth 2.0 Step-up Authentication Challenge Protocol

It is not uncommon for resource servers to require different
   authentication strengths or freshness according to the
   characteristics of a request.  This document introduces a mechanism
   for a resource server to signal to a client that the authentication
   event associated with the access token of the current request doesn't
   meet its authentication requirements and specify how to meet them.
   This document also codifies a mechanism for a client to request that
   an authorization server achieve a specific authentication strength or
   freshness when processing an authorization request.

[yuriyz]

Added "auth_time" and "acr" claims described by spec to introspection response and to access_token as JWT. AS identifies acr change and redirects to right authn script for re-authentication, in this way forcing step-up.