Remove more OmicronZoneConfig to BlueprintZoneConfig conversions

[jgallagher]

This builds on #5583; see it for rationale.

We remove half of the remaining callers that want to convert OmicronZoneConfig into BlueprintZoneConfig:

• nexus-test-utils's quasi-RSS: This now works in terms of BlueprintZoneConfig, and converts down to OmicronZoneConfig when necessary. This is the kind of transformation we'd like to make in real RSS, too, although it will be more complex there.
• One unit test in nexus-reconfigurator-execution: replaced using the representative() collection (which includes a collection from a live system, but no blueprint) with the example() system (which produces both a collection and a matching blueprint).
• Blueprint diffing - the types for the "before" side (which can be a collection or a blueprint) are now wrapped in two-variant enums, with some helpers for comparisons, instead of converting the collection to a blueprint.

The remaining users of this conversion direction are the most difficult, but at least we're down to three:

• nexus-db-model does this conversion to use some shared database serialization logic. This will necessarily change when we add to blueprints, since the serialization logic won't be shared anymore (or at least not as shared).
• RSS; see note above. We might be able to handle this like we did nexus-test-utils; I'll see when it becomes necessary.
• The DNS test that uses data from a live system.

Even if the last two have to keep this conversion direction for now (which will become more complex as data is added), at least it's only two! That should be manageable even if the process gets a little messy.