code: public ArmeriaRetrofitBuilder callbackExecutor(Executor executor) {
          callbackExecutor = requireNonNull(executor, "executor");
          retrofitBuilder.callbackExecutor(callbackExecutor);
          return this;
      }
docs: Sets the {@link Executor} on which {@link Callback} methods are invoked when returning {@link Call} from
  your service method.
  
  <p>Note: {@code executor} is not used for {@link #addCallAdapterFactory custom method return types}.
  
  @see Retrofit.Builder#callbackExecutor(Executor)