private Optional<ErrorResponse> createResponse(DiscFilterRequest request, int statusCode, String message) {
        request.setAttribute(RESULT_ERROR_CODE_ATTRIBUTE, statusCode);
        request.setAttribute(RESULT_ERROR_MESSAGE_ATTRIBUTE, message);
        if (passthroughMode) {
            return Optional.empty();
        } else {
            return Optional.of(new ErrorResponse(statusCode, message));
        }
    }
Authenticates Athenz principal through TLS client authentication (Athenz identity certificate).
The TLS authentication is based on the following assumptions:
 - The underlying connector is configured with 'clientAuth' set to either WANT_AUTH or NEED_AUTH.
 - The trust store is configured with the Athenz CA certificates only.

@author bjorncs
jdisc-security-filters/src/main/java/com/yahoo/jdisc/http/filter/security/athenz/AthenzPrincipalFilter.java