@UnstableApi
    default <U> FutureResponseAs<U> map(Function<T, U> mapper) {
        requireNonNull(mapper, "mapper");
        return response -> as(response).thenApply(mapper);
    }
Transforms the {@code T} type object into another by applying the {@link Function}.
core/src/main/java/com/linecorp/armeria/client/FutureResponseAs.java