Function authenticator 

pub async fn authenticator(
    credentials_path: &Path,
    token_path: &Path,
) -> Result<GoogleAuthenticator>

Build an authenticator for the given credential and token-cache paths.

The interactive loopback flow runs lazily on first token request (see obtain_token); constructing the authenticator alone does not prompt.

Errors

Returns Error::Auth when the client secret cannot be read or the authenticator cannot be built.