Simplify ObjectStore::List

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Having two layers of asynchrony and fallibility makes for a rather obnoxious interface. Removing this makes for better client ergonomics, at the expense of slightly more arcane logic to implement the ObjectStore

Describe the solution you'd like

I would like ObjectStore::list to synchronously return an async stream

Describe alternatives you've considered

Additional context

[tustvold]

label_issue.py automatically added labels {'object-store'} from #4930