FR: [AppCheck] Create token for AppCheck

Hello,

Currently, the Go Admin SDK only supports `VerifyToken` in App Check module and does not provide a way to create a new App Check token.
At the moment, only the [Node.js Admin SDK](https://firebase.google.com/docs/app-check/custom-provider) supports creating tokens:

```js
admin.appCheck().createToken(appId)
```

Could you please add support for CreateToken in the Go Admin SDK, similar to the Node.js Admin SDK? This would make it easier for Go applications to support App Check and improve application security.

Thank you.