我还有个疑问，苹果的 `refreshToken` 和 `access tokens` 是干嘛用的？文中说“which you use to obtain access tokens with future calls”，但是他也没有提供其他接口需要这个 `access tokens` 用来获取用户信息。
> Obtain a Refresh Token After verifying the identity token on your server, call the Generate and Validate Tokens endpoint with the client_id, client_secret, and nonce information.
On success, the server issues a refresh token, which you use to obtain access tokens with future calls. You may verify the refresh token up to once a day to confirm that the user’s Apple ID on that device is still in good standing with Apple’s servers. Apple’s servers may throttle your call if you attempt to verify a user’s Apple ID more than once a day.
If any step of the token verification fails, direct your app to fetch a new identity token for the user. Obtaining a new identity token on the device requires user interaction.