Feature Request: Support for service accounts with domain-wide delegation

G Suite allows you to [delegate domain-wide authority](https://developers.google.com/admin-sdk/directory/v1/guides/delegation) to a service account, which can be used for [sheets](https://www.googleapis.com/auth/spreadsheets.readonly) and [drive](https://www.googleapis.com/auth/drive.metadata.readonly). This seems to work in our internal scripts but not with the plugin. This would avoid the need to share every file with the service account.