[3/n] use update engine for reconfigurator execution

[sunshowers]

This starts to introduce the ability for the reconfigurator execution to be driven by the update engine. Doing so does add some complexity -- there's a new system driving updates -- but has several advantages that will be coming in future patches:

1. More fine-grained step reporting.
2. Progress reporting (not hooked up yet).

This is really basic so far -- for example, we don't currently report skipped steps, nor do we model nesting within the update engine. But it's a good place to get started, and we can keep enhancing this as time permits.

One change is that we now return and store a single anyhow::Error rather than a list of errors. If a step returns more than one error, we use a hack to combine the cause chains into a single message.

To serialize errors, we use the NestedError that's currently in update-engine. One benefit is that we now record the entire chain of error sources.

Within omdb, I also switched the blueprint executor display (and nothing else so far) to use tabled for easy alignment. This is what it looks like now:

task: "blueprint_executor"
  configured period: every 1m
  currently executing: no
  last completed activation: iter 110, triggered by a periodic timer firing
    started at 2024-08-27T05:36:57.748Z (7s ago) and ran for 227ms
    target blueprint:  70004e05-fc43-4534-ba51-ab0239b13f63
    execution:         enabled
    status:            failed at: Ensure dataset records (step 6/13)
    error:             step failed: Ensure dataset records
      caused by:       failed to insert dataset record for dataset 9d128b27-c9e9-4acf-b40b-44cbfbb0e2ba
      caused by:       Object (of type ById(6549a5fa-3f2e-4132-b2a1-56caf9028064)) not found: zpool

Depends on:

• [1/n] [reconfigurator] add a test wrapper around realize_blueprint_with_overrides #6444
• [2/n] [update-engine] add ( to the beginning of step indexes in the line displayer #6447
• omdb could interpret status of blueprint tasks #6440

[jgallagher]

LGTM, just a few nits/questions