Support mapping output type of ref:(...) as a reference

[ronaldtse]

There are occasions where a serialization is not meant to serialize child attributes as objects, but as references.

class Publication < Lutaml::Model::Serializable
  attribute :id, :string
  attribute :related_items, ref(Publication, :id), collection: true
end

books:
- id: doi-1234
  title: This is a Book
  related_items:
    - doi-2345
    - doi-3456
- id: doi-2345
  title: This is also a Book
  related_items:
    - doi-1234

This was last encountered when a reference to a parent was needed:

• Migrate to lutaml-model expressir#230

It makes me wonder if ref: is an "attribute value type" or a "mapping value type"?