[nexus] Add HostPhase1Updater

[jgallagher]

Continues the work from #4427 and #4502 adding types to Nexus that can deliver updates to components managed by the SP, and contains a similar amount of duplication from those two. Non-duplication changes are mostly refactoring:

• Removed component-specific error types in favor of SpComponentUpdateError
• Extracted the "start the update and poll MGS until it's done" logic into a new common_sp_update module

The test_host_phase1_updater_delivers_progress is subtly different from the RoT/SP versions of the same test, but not in a way that's particularly interesting (see the "Unlike the SP and RoT cases" comment for details).

[davepacheco]

I like the commonization here!