Token Revocation

This is a question more than a bug. It doesn't seem as though there is a way for a user to revoke an access token using googleapis_auth. I don't think this is a problem _per se_ because revoking tokens doesn't seem to be a typical procedure. However, my assumption was that any app that uses OAuth tokens should explicitly revoke them when the user signs out of the app. There is an [OAuth spec](https://oauth.net/2/token-revocation/) for this. Some [Google docs](https://cloud.google.com/apigee/docs/api-platform/security/oauth/enable-retrieval-and-revocation-oauth-20-access-tokens-end-user-id-app-id-or-both) seem to indicate that it's possible, but don't mention anything about best practices.

I'm just wondering if the dart APIs would benefit from code to revoke tokens...