Include exercise IDs in exercise download result data

[jolampi]

Desired behavior: download-or-update-course-exercises --exercise-id 98208 should return

{
  "output-kind": "output-data",
  "status": "finished",
  "message": "downloaded or updated exercises",
  "result": "executed-command",
  "data": {
    "output-data-kind": "exercise-download",
    "output-data": {
      "downloaded": [
        {
          "course-slug": "test-python-random-testcourse",
          "exercise-slug": "osa07-20_oma_ohjelmointikieli3",
          "path": "~/.../osa07-20_oma_ohjelmointikieli3",
          "id": 98208 // included here
        }
      ],
      "skipped": []
    }
  }
}