Produce configured tokenSource:
For internal use or for those programming around the Gmail 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
gm_auth() is what they need. If there is no current
gm_auth() is called to either load from cache or
initiate OAuth2.0 flow.
If auth has been deactivated via
request object (an S3 class provided by httr).