WIP: POC: Non recursive group export serialization

What does this MR do?

As part of the effort to introduce .ndjson on the group import/export feature, we need to reduce the number of nested structures. This is the initial effort to remove the nested groups on export.

Related to: #199430 (closed)

Currently the export is like:

{
  "name": "group1",
  /* attributes */
  "children": [
    {
      "name": "group2",
      /* attributes */
      "children": [
        {
          "name": "group3",
          /* attributes */
        }
      ]
    }
  ]
}

With the feature flag enabled the export will be like:

[
  {
    "name": "group2",
    "parent_id": 1,
    /* attributes */
  },
  {
    "name": "group3",
    "parent_id": 2,
    /* attributes */
  },
  {
    "name": "group1",
    "parent_id": null,
    /* attributes */
]

DO NOT MERGE

A re-architecture is being done on !26501 (merged), the non-recursive group serialization must be done on the new Writer abstraction. Until the new architecture is done, I'll work on this POC to be later implemented on the new abstractions.