public File uploadFile(String purpose, String filepath) {
        java.io.File file = new java.io.File(filepath);
        RequestBody purposeBody = RequestBody.create(MultipartBody.FORM, purpose);
        RequestBody fileBody = RequestBody.create(MediaType.parse("text"), file);
        MultipartBody.Part body = MultipartBody.Part.createFormData("file", filepath, fileBody);

        return execute(api.uploadFile(purposeBody, body));
    }
Creates a new OpenAiService that wraps OpenAiApi.
The ExecutorService must be the one you get from the client you created the api with
otherwise shutdownExecutor() won't work.
<p>
Use this if you need more customization.

@param api             OpenAiApi instance to use for all methods
@param executorService the ExecutorService from client.dispatcher().executorService()
service/src/main/java/com/theokanning/openai/service/OpenAiService.java