[FR] Allow to upload APNs auth keys to Firebase

**Is your feature request related to a problem? Please describe.**
We are trying to automate Firebase app creation since we manage many apps and most of them need push notifications. I played around with the Firebase admin SDK and got the most stuff working somehow, but I can't figure out how to upload the APNs auth key to Firebase. Is there and official way to do this via this SDK or via Rest API?

**Describe the solution you'd like**
I would like to have an SDK method or rest API endpoint where I can upload an Apple Push Notification service authentication key to Firebase so we can send push notifications to iOS apps managed in Firebase.

**Describe alternatives you've considered**
The alternative to having a method in the SDK would be at least a rest API endpoint.

**Additional context**
Funnily in my desperation I asked ChatGPT how it would solve this and it generated this code:
```js
const messaging = admin.messaging(app);
const apnsKey = {
authKey: '/path/to/AuthKey_XXXXXXXX.p8',
keyId: 'XXXXXXXX',
teamId: 'XXXXXXXX',
};

messaging.setApnsCredential(credential, apnsKey);
```

But I looked around the repo (even in older revisions) and could not find any method like this. Was there any and if yes, when and why was it removed?

首先查看 Firebase Admin Node.js SDK 的 messaging 接口和 Firebase REST API 文档，以确定是否支持上传 APNs 身份验证密钥；issue 未指定 repository 文件或测试。完成的标准是确定一条用于上传密钥的 SDK 或 REST API 路径，或清楚记录一种受支持的替代方案。

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