@Override
    public String getDefaultProxyHostname() {
        return delegate.getDefaultProxyHostname();
    }
{@link ArmeriaJaxrsClientEngine} will always be set as an {@link ClientHttpEngine}.
@throws UnsupportedOperationException always
resteasy/src/main/java/com/linecorp/armeria/client/resteasy/ArmeriaResteasyClientBuilder.java