@UnstableApi
    static FutureResponseAs<ResponseEntity<String>> string() {
        return aggregateAndConvert(AggregatedResponseAs.string());
    }
Aggregates an {@link HttpResponse} and converts the {@link AggregatedHttpResponse#content()} into
{@link String}.
core/src/main/java/com/linecorp/armeria/client/ResponseAs.java