Add CAS proxy tickets support to CasRealm

Add much fuller CAS support by adding CAS proxy chains and proxy ticket validations via standard CAS https callbacks for proxy tickets. This will enable to easily add features to CasRealm such as ClearPass support:

https://github.com/Unicon/cas-addons/wiki/Spring-Security-CAS-ClearPass-Extension

https://github.com/Unicon/cas-addons/blob/master/src/main/java/net/unicon/cas/addons/client/springsecurity/ClearpassRetrievingCasAuthenticationProvider.java