fm: want PSCs to exist in the external API

[hawkw]

Presently, we have /system/hardware/sleds and /system/hardware/switches APIs for fetching and listing sleds and switches, but there is no corresponding API for power shelves/power shelf controllers. We will need such an API to give fault management the language to refer to Active Problems that impact a power shelf.

I'd want a PSC API model to include:

• The PSC's baseboard identity (part, revision, serial`)
• An immutable system-assigned UUID
• The rack UUID
• Which physical power shelf slot it's in (shelf 0/shelf 1)

This should be pretty straightforward to add, since the baseboard identity is already in the inventory along with the MGS slot it was inventoried from.

[hawkw]

@rmustacc proposes that we might really want to model this as an API that lists power shelves, with the PSC and PSUs (see #11047) presented as children of the shelf that they're inside of. This would be in contrast to a model where the PSC "owns" the PSUs and just also says which shelf number it's in. I agree that this is more conceptually correct, though it feels a little bit goofy to assign a UUID and so forth to the power shelf itself, which I think is best thought of as a location rather than an entitiy (it has no VPD identity that I'm aware of)...hmm.