@UnstableApi
    public <T> FutureTransformingRequestPreparation<ResponseEntity<T>> asEntity(
            FutureResponseAs<ResponseEntity<T>> responseAs) {
        requireNonNull(responseAs, "responseAs");
        return new FutureTransformingRequestPreparation<>(this, responseAs);
    }
Sets the specified {@link ResponseAs} that converts the {@link HttpResponse} into
a {@link ResponseEntity}.
core/src/main/java/com/linecorp/armeria/client/WebClientRequestPreparation.java