CAS-1238 Develop authn policy driven by registered service meta data.

Enhanced RegisteredService with getRequiredHandlers() getter to define
the set of authentication handlers that must successfully authenticate
credentials in order to access the corresponding service.
RequiredHandlerAuthenticationPolicyFactory is the corresponding policy
component injected into CentralAuthenticationServiceImpl to enforce the
security constraints.

AutenticationPolicy was extended to ContextualAuthenticationPolicy so it
exposes the context information used to drive the policy. This provides an
easy facility to retrieve and store the context information in
UnsatisfiedAuthenticationPolicyException on policy failure. In this way
the context information could be exposed to higher layers of the
application in order to route the user to detailed error views that could
provide suggestions for corrective action.