code: public PubsubFuture<SubscriptionList> listSubscriptions(final String project) {
      final String path = "projects/" + project + "/subscriptions";
      return get("list subscriptions", path, readJson(SubscriptionList.class));
    }
docs: List the Pub/Sub subscriptions in a project. This will get the first page of subscriptions. To enumerate all
  subscriptions you might have to make further calls to {@link #listTopics(String, String)} with the page token in
  order to get further pages.
  
  @param project The Google Cloud project.
  @return A future that is completed when this request is completed.