EntityReferenceItem should support a virtual `target_uuid` property

I realize that since this is about the field items rather than the field itself, it's a default content problem rather than a config-dependency-on-content problem, but really, it's kinda also the same problem space. However it's implemented, ER should be able to reference a UUID somehow, as part of the core API.

Adding ✨ Add a command-line utility to export content in YAML format Active as related because this would allow us to remove some of the special processing from the ContentExportNormalizer we're adding in that issue.