fm: want power shelf PSU inventory in the external API

Once we have external API endpoints for listing/viewing PSCs (#11045), we should also have a way to publicly reference the inventory of PSUs in each power shelf. Looking at existing APIs for reference, I notice that physical disks can either be queried as a list of all physical disks or as a list of physical disks attached to a given sled, so we might want to do something similar for rectifiers.

The API model for a PSU should include at least:

• a stable UUID that uniquely identifies that PSU
• the PSU's VPD (this will require oxidecomputer/hubris#2212 so that we can actually inventory PSU VPD from the PSC)
• its physical slot within the power shelf
• which power shelf it is in (either by referencing the PSC by UUID, by explicitly saying the shelf number, or both?)

No repository files or tests are named. Start by reviewing issue #11045 and the linked physical-disk API examples, then check the Hubris dependency #2212 for PSU VPD inventory; the work is done when the external API exposes PSU UUID, VPD, physical slot, and power-shelf association.