Add support for authenticating with `CLOUDSDK_AUTH_ACCESS_TOKEN`

[sbakiu]

In my organization we are using temporary access tokens and it would be very helpful to have the authentication via CLOUDSDK_AUTH_ACCESS_TOKEN environment variable supported.

Probably related discussion on the google api auth library: googleapis/google-cloud-python#15199

[michaelCTS]

Just ran into this issue and took a while to debug. The problem is that gauth.py first tries to use an API approach to get credentials and only falls back to gcloud (which respects CLOUDSDK_AUTH_ACCESS_TOKEN) when that fails. However, authentication succeeds when in environments where metadata.google.internal can be contacted! Credentials are automatically retrieved and CLOUDSDK_AUTH_ACCESS_TOKEN thus ignored.

Would be great if priority were given to CLOUDSDK_AUTH_ACCESS_TOKEN, then gcloud.