Decommissioning expunged disks should happen in planner

[andrewjstone]

Currently, we decommission expunged disks during reconfigurator execution. This expungement step is tied to a previous step in the executor that ensures that any expunged disks no longer exist on the given sled.

Unfortunately, this coupling prevents us from making all execution steps independent and skippable as a solution to #6999.
We can resolve this issue by decommissioning disks in the planner, rather than in the executor as we do with sled decommissioning

[andrewjstone]

In order to have the planner do this, it needs to learn about the physical disks that the sled-agent has via inventory. These physical disks are already retrieve in inventory. However they don't include the generation number of the current configuration on the sled-agent. We likely want to return that along with the configuration so that we don't inadvertently decommission disks that have not yet been expunged by the executor.

[andrewjstone]

physical disk generation number added to the inventory in #7162