@Override
    protected String extractPrincipalId(final Credential credential, final Optional<Principal> currentPrincipal) {
        val c = (SpnegoCredential) credential;
        return c.getPrincipal().getId();
    }
Implementation of a CredentialToPrincipalResolver that takes a
SpnegoCredential and returns a SimplePrincipal.

@author Arnaud Lesueur
@author Marc-Antoine Garrigue
@since 3.1
support/cas-server-support-spnego/src/main/java/org/apereo/cas/support/spnego/authentication/principal/SpnegoPrincipalResolver.java