Create a Google APIs OAuth2.0 credentials object
GoogleApiCreds(userName = Sys.getenv(paste0(appname, "_USER")),
appCreds = NULL, cache = character(0), use_oob = FALSE,
appname = "GOOGLE_APIS")
Google username email address hint
Filename or named vector for client_id and client_secret.
httr OAuth2.0 cache
as per httr
prefix of environment variables that hold the client ID and client secret.