`omdb` command to better summarize what update is currently doing

The Reconfigurator ops guide describes how to figure out what's going on when an update is stuck:
https://github.com/oxidecomputer/omicron/blob/main/docs/reconfigurator-ops-guide.adoc#debug-stuck

but it does involve a few different commands to query a few different components. It'd be handy to have a command like omdb reconfigurator update-activity (that's a terrible name) that reports a bunch of it together. It could report:

• whether an update is in progress
• similar output as update-status: counts of components at various versions
• the last blueprint (or the last few), what time they were, and what they did (i.e., their comment and/or a diff)
• (a warning if the last blueprint was too long ago and the update is not finished)
• any errors or warnings from relevant tasks: blueprint planner, blueprint execution, rendezvous, etc.
• the latest planning report from some Nexus
• the latest execution report from some Nexus

It's worth spending some time figuring out how to best present this information so that it's as clear as possible what the current status is.

Start with docs/reconfigurator-ops-guide.adoc#debug-stuck and the existing update-status command to identify the current queries and components involved. Determine how omdb reconfigurator update-activity should combine update progress, blueprint history, task warnings, and Nexus reports. Done means the command presents the relevant current status clearly in one place.