CTF-next: use union for reference fields with validation

[axe312ger]

The current state allows you to query all content types in a reference field, even when the user/editor can not select it based on Contentful's field validations.

This change will adjust the schema to reflect the decisions made in the Contentful reference field validations.

Note: This is blocked from merging till Contentful CDA API delivers single reference field validations. A patch for the API is in progress :)

[axe312ger]

Unfortunately field validations are not exposed for single reference fields. This blocks us from finishing this feature.

I escalated this problem to the Contentful API team. Will keep u posted if I hear back.

Here is a example response from CDA content-types:

{
  "id": "oneItemSingleType",
  "name": "One Item: Single Type",
  "type": "Link",
  "localized": false,
  "required": false,
  "disabled": false,
  "omitted": false,
  "linkType": "Entry"
}
{
  "id": "oneItemManyTypes",
  "name": "One Item: Many Types",
  "type": "Link",
  "localized": false,
  "required": false,
  "disabled": false,
  "omitted": false,
  "linkType": "Entry"
}
{
  "id": "oneItemAllTypes",
  "name": "One Item: All Types",
  "type": "Link",
  "localized": false,
  "required": false,
  "disabled": false,
  "omitted": false,
  "linkType": "Entry"
}
{
  "id": "multipleItemsSingleType",
  "name": "Multiple Items: Single Type",
  "type": "Array",
  "localized": false,
  "required": false,
  "disabled": false,
  "omitted": false,
  "items": {
    "type": "Link",
    "validations": [
      {
        "linkContentType": [
          "text"
        ]
      }
    ],
    "linkType": "Entry"
  }
}
{
  "id": "multipleItemsManyTypes",
  "name": "Multiple Items: Many Types",
  "type": "Array",
  "localized": false,
  "required": false,
  "disabled": false,
  "omitted": false,
  "items": {
    "type": "Link",
    "validations": [
      {
        "linkContentType": [
          "number",
          "text"
        ]
      }
    ],
    "linkType": "Entry"
  }
}
{
  "id": "multipleItemsAllTypes",
  "name": "Multiple Items: All Types",
  "type": "Array",
  "localized": false,
  "required": false,
  "disabled": false,
  "omitted": false,
  "items": {
    "type": "Link",
    "validations": [],
    "linkType": "Entry"
  }
}

[axe312ger]

We got a patch ready that exposes the field validation for single reference fields.

I'll merge this PR into #30855 as soon we released the updated API to the public.

[axe312ger]

Looks good now. Will add tests and merge into main PR :)