GithubImporter: Expose import stats on import status endpoint

What does this MR do and why?

When checking the import status for a github import project, now the user will be able to see the import current stats, like:

{
  "id": 27,
  "description": null,
  "name": "gh_30086",
  "name_with_namespace": "Administrator / gh_30086",
  "path": "gh_30086",
  "path_with_namespace": "root/gh_30086",
  "created_at": "2021-12-07T17:45:44.870Z",
  "import_status": "finished",
  "import_type": "github",
  "correlation_id": "01FPB00M3VX3SGVQYGBPBY6QCH",
  "failed_relations": [],
  "import_error": null,
  "stats": {                              // <=== This is the new field
    "fetched": {
      "diff_note": 19,
      "issue": 3,
      "label": 1,
      "note": 3,
      "pull_request": 2,
      "pull_request_merged_by": 1,
      "pull_request_review": 16
    },
    "imported": {
      "diff_note": 19,
      "issue": 3,
      "label": 1,
      "note": 3,
      "pull_request": 2,
      "pull_request_merged_by": 1,
      "pull_request_review": 16
    }
  }
}

Related to: #347433 (closed)

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