FR: Firebase Auth provider-level OIDC claims request support

### Operating System

Windows 10

### Environment (if applicable)

Chrome 149, Node v24.18.0

### Firebase SDK Version

12.15.0

### Firebase SDK Product(s)

Auth

### Project Tooling

N/A

### Detailed Problem Description

Firebase Auth Web currently exposes `GoogleAuthProvider.addScope()` and `GoogleAuthProvider.setCustomParameters()`, but there does not appear to be a supported way to configure an OpenID Connect `claims` request on `GoogleAuthProvider`.

That makes it difficult to use Google Sign-In security signals that require explicit claims requests while still staying within Firebase Auth’s `signInWithPopup` / `signInWithRedirect` / link / reauthenticate abstractions.

Google documents additional security signals for Sign in with Google, including OIDC claims such as `auth_time` and `amr`.
These are relevant for risk-based account decisions and step-up / recent-auth / authentication-strength checks.

Examples from Google docs:
- `auth_time`
- `amr`

In Google Cloud / Google Identity surfaces, this is related to options such as:
- Session duration claim
- Authentication strength claim

### Current limitation

With the current Firebase Web Auth API, I can do this:

```ts
const provider = new GoogleAuthProvider();
provider.addScope('profile');
provider.addScope('email');
await signInWithPopup(auth, provider);
```

But there is no supported API to express an OIDC claims request like:
```
{
"id_token": {
"auth_time": { "essential": true },
"amr": { "essential": true }
}
}
```

setCustomParameters() also does not seem like the right fit here, since this is not just a simple login hint / prompt parameter. It is a structured OIDC claims request.

**Request**
Please consider adding provider-level support for OIDC claims requests on GoogleAuthProvider (or a suitable shared OAuth/OIDC provider base class).

Not prescribing the exact API, but something conceptually like:
```ts
const provider = new GoogleAuthProvider();
provider.setRequestedIdTokenClaims({
auth_time: { essential: true },
amr: { essential: true },
});
```

**Why this would help**
Right now the workaround is to leave Firebase’s built-in Google popup/redirectflow, perform the Google OIDC/GIS flow manually, and then bridge back intoFirebase with signInWithCredential().
That works, but it adds avoidable complexity for apps that otherwise want tostay inside Firebase Auth’s built-in provider flows.

**Additional context/notes**
I can take an incremental approach to ensure stability.
Is this currently on your radar or part of the future roadmap?
Can I make a PR and contribute to this part? If possible, I would like to proceed with this work.
I’ll ensure it works seamlessly with the existing API without breaking changes.

References
[Google Sign-In security bundle docs](https://developers.google.com/identity/siwg/security-bundle)
[Firebase Auth Web Google sign-in docs](https://firebase.google.com/docs/auth/web/google-signin)

### Steps and code to reproduce issue

N/A

Start by reviewing the GoogleAuthProvider entry point and its existing addScope() and setCustomParameters() methods, then trace how signInWithPopup(), signInWithRedirect(), linking, and reauthentication pass provider parameters. Define the provider-level OIDC claims API and verify that claims survive each supported flow without breaking existing behavior.

Written by the indexing model from the issue text.