roachprod: override labels to support extend for MIGs

Currently the extend command updates the labels on VMs to extend the lifetime of a cluster. This works on a MIG (managed instance group) cluster as well. But when adding new nodes to a MIG cluster on roachprod it will inherit the labels the instance template for the cluster originally had, and will not automatically inherit the new updated labels that were modified via extend.

To solve this problem, GCE does provide a way of overriding labels originally specified on the instance template. This functionality should work well to enable extend & grow to work together.

See #118207 for more details.

Epic: [CRDB-33832](https://cockroachlabs.atlassian.net/browse/CRDB-33832)

Jira issue: CRDB-36048