Add ability to use service account impersonation.
As part of https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-product-factory/-/issues/1 we want to use service account impersonation exclusively to remove the use of service account key files. This MR adds the required option to logan to pass through the current user's ~/.config/gcloud/application_default_credentials.json
to use when impersonating.