For internal use or for those programming around the Sheets API. Returns a token pre-processed with httr::config(). Most users do not need to handle tokens "by hand" or, even if they need some control, gs4_auth() is what they need. If there is no current token, gs4_auth() is called to either load from cache or initiate OAuth2.0 flow. If auth has been deactivated via gs4_deauth(), gs4_token()