compute: Allow postgres user to power off the VM (#10710)

I plan to use this when launching a fast_import job in a VM. There's
currently no good way for an executable running in a NeonVM to exit
gracefully and have the VM shut down. The inittab we use always respawns
the payload command. The idea is that the control plane can use
"fast_import ... && poweroff" as the command, so that when fast_import
completes successfully, the VM is terminated, and the k8s Pod and
VirtualMachine object are marked as completed successfully.

I'm working on bigger changes to how we launch VMs, and will try to come
up with a nicer system for that, but in the meanwhile, this quick hack
allows us to proceed with using VMs for one-off jobs like fast_import.

compute/vm-image-spec-bookworm.yaml

@@ -47,7 +47,9 @@ files:
       # Allow postgres user (which is what compute_ctl runs as) to run /neonvm/bin/resize-swap
       # and /neonvm/bin/set-disk-quota as root without requiring entering a password (NOPASSWD),
       # regardless of hostname (ALL)
-      postgres ALL=(root) NOPASSWD: /neonvm/bin/resize-swap, /neonvm/bin/set-disk-quota
+      #
+      # Also allow it to shut down the VM. The fast_import job does that when it's finished.
+      postgres ALL=(root) NOPASSWD: /neonvm/bin/resize-swap, /neonvm/bin/set-disk-quota, /neonvm/bin/poweroff
   - filename: cgconfig.conf
     content: |
       # Configuration for cgroups in VM compute nodes