reconfigurator-cli could let you set SP versions

[davepacheco]

Depends on #8269.

This PR:

• augments reconfigurator-cli's internal model of sleds to include cabooses for the SP's active and inactive slots
  • for simulated models: fills in arbitrary defaults
  • for models loaded from a saved file: loads data out of the saved inventory
• shows the sled serial number and these new versions in the output of sled-show
• includes these cabooses in inventory collections generated by inventory-generate
• adds a command sled-update-sp for setting the version in these caboose fields
• adds tests for all this

The goal here is to work toward being able to demo/test the planner behavior around pending MGS updates. With this PR (and #8269, on which it's based), I was able to:

• generate a blueprint with a configured MGS update
• demo behavior of seeing that an update has become impossible:
  • use sled-update-sp to set the inactive slot to something else, invalidating the configured update
  • run the planner and see it remove the configured update because it's impossible. (It doesn't create a replacement yet because this requires teaching reconfigurator-cli about TUF artifacts, which it doesn't know about yet.)
• demo behavior of seeing that an update has finished:
  • use sled-update-sp to set the active slot to the version we're trying to update to
  • run the planner and see it remove the configured update because it's done

There are some limitations here:

• You can't set/demo/test anything about switch or PSC versions because reconfigurator-cli has no model of these.
• You can't get the planner to configure a new update because reconfigurator-cli doesn't know about TUF artifacts. (This does work after reconfigurator-cli should support setting target release #8283.)

[karencfv]

Looks great! Just a small optional nit