firebase serve not using gcloud impersonated_service_account

I am not sure this should be reported bug or feature request but I think `firebase serve` functions should allow using `createCustomToken` without the need for `GOOGLE_APPLICATION_CREDENTIALS` set

I have already try using `gcloud auth application-default login` with `--impersonate-service-account` and it seem firebase serve are not using this credential for firebase auth functions

If this is not a bug I would like to request this feature that firebase tools should automatically request for `impersonated_service_account` at firebase serve. It was more secure to require login with account in local development environment without the need to download private key and linked it within project folder