@Override
    protected String extractPrincipalId(final Credential credential, final Optional<Principal> currentPrincipal) {
        return ((PrincipalBearingCredential) credential).getPrincipal().getId();
    }
Extracts the Principal out of PrincipalBearingCredential. It is very simple
to resolve PrincipalBearingCredential to a Principal since the credentials
already bear the ready-to-go Principal.

@author Andrew Petro
@since 3.0.0
support/cas-server-support-trusted/src/main/java/org/apereo/cas/adaptors/trusted/authentication/principal/PrincipalBearingPrincipalResolver.java