want API for looking up sleds by which slot in the rack they're in

#11075 updates the /system/hardware/sleds APIs to include the slot (cubby) number the sled was last seen in. It would be nice to also have a way to go in the opposite direction: given a slot number (and rack ID, because multirack), look up the sled in that slot (or indicate that no sled is present in that slot).

This probably shouldn't just return a Sled model though, because there's a third state besides "sled present and part of the cluster" and "sled not present", which is "sled physically present but not part of the cluster". We would probably want this API to also check ignition to see if there's anything in that cubby, even if there is not a record in the sled table for it. So we'd need to do some more API design here.

I think we'd probably want an API like this to hang off of /system/hardware/racks, because in a multi-rack control plane, a slot number only makes sense within a given rack. So something like:

/v1/system/hardware/racks/{rack_id}/sleds/{sled_number}

Start by reviewing issue #11075 and the existing /system/hardware/sleds APIs, then examine how sled records and ignition presence are represented. Design the rack-scoped endpoint using rack_id and sled_number, covering clustered sleds, empty slots, and physically present sleds without cluster records; done means the API behavior and response states are defined and implemented.