List Holiday Categories

curl --request GET \
  --url https://api.zeeg.me/v2/holidays/categories \
  --header 'Authorization: Bearer <token>'

{
  "collection": [
    {
      "key": "DE",
      "name": "Germany",
      "regions": [
        {
          "key": "DE-BY",
          "name": "Bavaria"
        },
        {
          "key": "DE-BE",
          "name": "Berlin"
        }
      ]
    },
    {
      "key": "FR",
      "name": "France",
      "regions": []
    }
  ]
}