[Feature Request] Support App Check for Firebase Authentication beta

It seems like recently App Check became available to use with Firebase Authentication as beta:

However, currently the `X-Firebase-Appcheck` header is missing from requests made by the `Auth` class (`from "@angular/fire/auth"`), causing them to be classified as unverified.

## Versions Info
- **angular:** `16.2.2`
- **@angular/fire**: `7.6.1`

先追蹤 Auth 請求如何在 @angular/fire/auth 進入點中組合，以及 Firebase App Check 如何整合。確認預期的 beta 行為，並找出現有的請求測試；完成的標準是 Auth 請求包含 X-Firebase-Appcheck 標頭，且該行為有測試涵蓋。

由索引模型根據 Issue 內容生成。