public ArmeriaRetrofitBuilder callbackExecutor(Executor executor) {
        callbackExecutor = requireNonNull(executor, "executor");
        retrofitBuilder.callbackExecutor(callbackExecutor);
        return this;
    }
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)
retrofit2/src/main/java/com/linecorp/armeria/client/retrofit2/ArmeriaRetrofitBuilder.java