firebase_auth_desktop throws UnimplementedError on tenantId

## Bug report

**Describe the bug**

Firebase's User.toString() method looks like this:

```
@override
String toString() {
return '$User('
'displayName: $displayName, '
'email: $email, '
'emailVerified: $emailVerified, '
'isAnonymous: $isAnonymous, '
'metadata: $metadata, '
'phoneNumber: $phoneNumber, '
'photoURL: $photoURL, '
'providerData, $providerData, '
'refreshToken: $refreshToken, '
'tenantId: $tenantId, ' // <<<<< Note!
'uid: $uid)';
}
```

This calls tenantId from the user in firebase_auth_desktop, which forwards to the user from firebase_auth_dart where tenantId is unimplemented:

```
/// The current user's tenant ID.
///
/// This is a read-only property, which indicates the tenant ID used to sign in
/// the current user.
/// This is null if the user is signed in from the parent project.
String? get tenantId => throw UnimplementedError();
```

### Steps to reproduce

Steps to reproduce the behavior:

1. Call User.toString() on a Firebase User

### Expected behavior

The toString() method should work and tenantId should appear.

### Flutter doctor

Run `flutter doctor` and paste the output below:

Click To Expand

```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 2.10.5, on Fedora Linux 37 (Workstation Edition) 6.0.16-300.fc37.x86_64, locale en_GB.UTF-8)
[✓] Android toolchain - develop for Android devices (Android SDK version 32.1.0-rc1)
[✓] Linux toolchain - develop for Linux desktop
[!] Android Studio (not installed)
[✓] VS Code (version 1.74.2)
[✓] Connected device (1 available)
[✓] HTTP Host Availability
```