Entity reference field normalization has target_type and target_uuid, but not used in denormalization

#2060677: Add target_type, target_uuid to serialized output of entity reference fields in non-HAL formats added target_type and target_uuid to the normalization. But didn't add denormalization support. It was intended to become a follow-up, but that follow-up was never filed. Until now.

#2060677-21: Add target_type, target_uuid to serialized output of entity reference fields in non-HAL formats:

Should we also be adding a new EntityResolverInterface here for denormalizing?

#2060677-22: Add target_type, target_uuid to serialized output of entity reference fields in non-HAL formats:

We have the right tools already I think, we just need serialization module to actually use them. I think we should do that in a follow up to this though, and just keep this for adding this to the denormalized data. Sound ok?