feat(auth): Added multi-tenancy support

[hiranya911]

Merging the tenant-mgt branch into main.

RELEASE NOTE: Added TenantManager class, which provides APIs for creating, updating, retrieving and deleting tenants in GCIP-enabled Firebase projects. RELEASE NOTE: Added TenantAwareFirebaseAuth class, which provides APIs for tenant-scoped user management, token verification and IdP configuration management. RELEASE NOTE: Added TenantNotFound and TenantIdMismatch error codes to the AuthErrorCode enum. RELEASE NOTE: Added the nullable property TenantId from UserRecord and FirebaseToken classes.

Resolves #149

[hiranya911]

@kevinthecheung requesting API reference review for following classes:

• Tenant
• TenantArgs
• TenantManager
• TenantAwareFirebaseAuth
• TenantsPage
• ListTenantsOptions
• AuthErrorCode