[FR] Use firebase emulator for TenantManagementService

[Jaakkonen]

Is your feature request related to a problem? Please describe.
Currently only the Auth client supports running against the firebase emulator when setting the FIREBASE_AUTH_EMULATOR_HOST environment variable. However when wanting to list for example tenants with TenantManagementService you cannot do that and it forcibly uses TENANT_MGT_URL = 'https://identitytoolkit.googleapis.com/v2'. The emulator itself however emulates some of these endpoints (like /projects/{project_id}/tenants) so some of those are supported.

Describe the solution you'd like
Add support for setting FIREBASE_TENANT_EMULATOR_HOST and use that in _TenantManagementService

Describe alternatives you've considered
Doing direct HTTP REST API calls to the emulator or extending the management service manually like

class EmulatedTenantManagementService(tenant_mgt._TenantManagementService):
    TENANT_MGT_URL = (
        f"http://{firebase_auth_emulator_host}/identitytoolkit.googleapis.com/v2"
    )

    def __init__(self, app: firebase_admin.App):
        super().__init__(app)
        credential = _utils.EmulatorAdminCredentials()
        version_header = f"Python/Admin/{firebase_admin.__version__}"
        base_url = f"{self.TENANT_MGT_URL}/projects/{app.project_id}"
        self.client = _http_client.JsonHttpClient(
            credential=credential,
            base_url=base_url,
            headers={"X-Client-Version": version_header},
        )

app = firebase_admin.get_app()
service = EmulatedTenantManagementService(app)
app._services[tenant_mgt._TENANT_MGT_ATTRIBUTE] = service