Create a union type for drafts / published documents in GraphQL

[jmikrut]

Right now, if you have drafts enabled, it's possible to save a partially completed document where no fields are required. We do this so that you can complete a document slowly, over time, without having to fill out all required fields before saving the first time.

But there is an unfortunate side-effect here - all fields are nullable with drafts enabled because of this.

To solve this, we could create a union type for collections with drafts enabled: type Page = PagePublished | PageDraft where we could enforce non-nullable fields as required on the Published type, but keep them nullable on the Draft type.

Secondly, as a backup plan, we could do one of the following:

1. Create an option to disable the saving of incomplete drafts, which would force the user to complete a full copy of the document before saving a draft
2. Create additional queries that automatically only return _status: 'published' documents, wherein we restrict documents that are sent out internally, therefore guaranteeing that all completed fields have indeed been completed and are present.

Tracking this for visibility here - we'll add a solution to the roadmap accordingly once we determine the best way forward!

Related: #5016