public void shutdownExecutor() {
        Objects.requireNonNull(this.executorService, "executorService must be set in order to shut down");
        this.executorService.shutdown();
    }
Shuts down the OkHttp ExecutorService.
The default behaviour of OkHttp's ExecutorService (ConnectionPool)
is to shut down after an idle timeout of 60s.
Call this method to shut down the ExecutorService immediately.
service/src/main/java/com/theokanning/openai/service/OpenAiService.java