[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`

Start by tracing how Auth requests are assembled in the @angular/fire/auth entry point and how Firebase App Check is integrated. Confirm the expected beta behavior and identify existing request tests; done means Auth requests include the X-Firebase-Appcheck header and the behavior is covered by tests.

Written by the indexing model from the issue text.