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, sheets_auth() is what they need. If there is no current
token, sheets_auth() is called to either load from cache or
initiate OAuth2.0 flow.
If auth has been deactivated via sheets_deauth(), sheets_token()
returns NULL.