Add tenantId Support in Firebase C++ SDK

**Description:**
The Firebase C++ SDK does not expose the tenantId property on the user or auth instance, unlike other Firebase SDKs. This omission makes it impossible to implement multi-tenancy, a critical requirement for SaaS and multi-tenant applications.

**Impact:**
- Developers using the C++ SDK face limitations in building multi-tenant applications.
- Inconsistency with other Firebase SDKs creates unnecessary challenges.

**Reference:**
This issue is similar to the tenant management gap noted in [flutterfire/issues/13500](https://github.com/firebase/flutterfire/issues/13500).

**Proposed Solution:**
Add tenantId as a property to the user and auth instance in the Firebase C++ SDK.