Manage compute_engine.Credentials default service_account_email case

[loicsikidi]

Context

When the library runs on GCP infrastructure, compute_engine.Credentials is used by google.application.default().

For some strange reason, the value given to service_account_email property is default (see more here).

The latter is updated (with the correct value) only after the refresh process (see more here).

The bug is only observable in the following use-case (when the target_principal is deducted from the source_credentials):

source_credentials, _ = google.auth.default()

delegated_credentials = dwd_credentials.Credentials(
  subject=subject,
  source_credentials=source_credentials,
  target_scopes = target_scopes,
)

The lib must handle this scenario!