Make config builders, not validated structs, [de]serialize

This completes #371 (closed) and is the last work item from #285 (closed) that isn't localised (to a particular crate or layer, or to a particular area of configuration).

assigned to @Diziet

requested review from @nickm

LGTM! It's been a lot of work leading up to this point, and it's great to see it finally reach completion.

merged

mentioned in commit 5b9d71f7

mentioned in issue #371 (closed)