@UnstableApi
    public RestClient restClient(Consumer<WebClientBuilder> webClientCustomizer) {
        requireNonNull(webClientCustomizer, "webClientCustomizer");
        return delegate.restClient(webClientCustomizer);
    }
Returns a newly created {@link RestClient} configured by
{@link #configureWebClient(WebClientBuilder)} and then the specified customizer.
junit5/src/main/java/com/linecorp/armeria/testing/junit5/server/ServerExtension.java