[sled-agent-config-reconciler] Add DumpSetupTask

[jgallagher]

Adds a tokio task responsible for activating DumpSetup whenever the set of disks changes. This is analogous to the current StorageMonitor, with two mechanical changes:

• It no longer makes sense to wait for changes to generation, because that covers the entire config (and usually will be bumped just for changes to the zones; e.g., during updates). We instead consume channels that contain the managed disks and keep our own "what was the last set of disks we used" to decide when to forward a change notification on to update_dumpdev_setup.
• We consume two channels due to the split of internal vs external disks within the config reconciler.

Staged on top of #8140.