Discovered when working on IBX-6476.

Steps to reproduce

1. Create a content type with enabled configured SEO field (see IBX-6476, IBX-5931). You can use backoffice as it's not relevant for the issue
2. Create a content item of that type and set some values in SEO section
3. Try loading that content item through GET /api/ibexa/v2/content/objects/<content_id>.

Actual result

Observe that the returned response, e.g. in json format lacks set SEO values

{
  "Content": {
    "CurrentVersion": {
      "Version": {
        "Fields": {
          "field": [
            {
              "id": 296,
              "fieldDefinitionIdentifier": "seo_field",
              "languageCode": "eng-GB",
              "fieldTypeIdentifier": "ibexa_seo",
              "fieldValue": ""
            }
          ]
        },
      }
    },   
}

Expected result

Set SEO values are present in REST response.

Note: most likely it's also impossible to set those values when creating content via REST - TBD.