Incorporate rack ID into inventory collection system

The inventory collection code doesn't currently save the rack ID of the various components it tracks. For multi-rack, we will have to add this.

We may not need to add a rack ID to every type of hardware being tracked, but at a minimum we'll need to track it for the SPs; this will be necessary to address the multi-rack TODO in Nexus::sled_list_uninitialized. That said, there may be other uses of the inventory code where we'll need access to rack IDs as well, and I'll plan to add them to this issue if/when I find more. And even if there aren't any more reasons we need to track rack IDs right now, it might be a good idea to add a rack ID to more parts of the inventory system than just the SPs, assuming it's easy to do.

Note that this depends on #5201: the inventory collection code grabs the SP data from MGS, so we'll need a way to connect to other racks' MGS instances.

Start by tracing the inventory collection code and the SP data path through MGS, then inspect the multi-rack TODO in Nexus::sled_list_uninitialized. Review dependency #5201 to understand how other racks' MGS instances will be reached. Done means SP inventory includes rack IDs and the multi-rack path can use them; determine whether other hardware types also need them.