Future: add "user-license" GCP features to pycloudlib

[renanrodrigo]

See --update-user-license feature here https://cloud.google.com/sdk/gcloud/reference/beta/compute/disks/update?hl=en

Notice that it is a "beta" feature only at time of writing, which may (or may not?) complicate things.

This would help the Pro Client team in writing and maintaining integration/acceptance tests about GCP in-place upgrades.