Add tenantId Support in Firebase C++ SDK

[itsthemanish]

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.

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

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[jonsimantov]

Thanks - we'll look into this.