add information about SP-related updates to blueprint

[davepacheco]

This is a first cut of adding information about SP-related updates to the blueprint.

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(copied from #7741 and edited because this is basically an update of that PR)

The plan is that the blueprint will maintain a list of pending MGS-managed updates. An MGS-managed update is any update done by the SP and controlled via MGS, which means: RoT bootloader, RoT Hubris image, SP Hubris image, and host phase 1. There may be at most one of these pending for any baseboard at once, but there could be multiple baseboards getting an update at once. (A baseboard is a thing with an SP on it: Gimlet, Sidecar, or PSC.)

When the planner wants to update one of these things, it will add a PendingMgsUpdate to the (next) blueprint. When it notices that the inventory reflects that that change has been made, it removes it from the (next) blueprint.

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See #7976 for the bigger picture (PRs ahead of and behind this one).

Depends on #7974.

[jgallagher]

LGTM, mostly just questions about followup work

[davepacheco]

Thanks for the review! I'm tracking all the follow-up work in the Reconfigurator board but I'm happy to make sure real (non-draft) issues are filed and maybe note them in the comments.