add attribute `additional` to additional stopovers with test

[dabund24]

As described in #303, a stopover now has an attribute additional if it is additional (i.e. it is a "Zusatzhalt"). Also includes a test checking this behavior.

Example:

{
  "stop": {
    "type": "stop",
    "id": "8000156",
    "name": "Heidelberg Hbf",
    "location": {
      "type": "location",
      "id": "8000156",
      "latitude": 49.403582,
      "longitude": 8.67548
    },
    "products": {
      "nationalExpress": true,
      "national": true,
      "regionalExpress": true,
      "regional": true,
      "suburban": true,
      "bus": true,
      "ferry": false,
      "subway": false,
      "tram": true,
      "taxi": true
    }
  },
  "arrival": "2023-11-25T15:49:00+01:00",
  "plannedArrival": "2023-11-25T15:34:00+01:00",
  "arrivalDelay": 900,
  "arrivalPlatform": "7a",
  "arrivalPrognosisType": "prognosed",
  "plannedArrivalPlatform": null,
  "departure": "2023-11-25T15:50:00+01:00",
  "plannedDeparture": "2023-11-25T15:35:00+01:00",
  "departureDelay": 900,
  "departurePlatform": "7a",
  "departurePrognosisType": "prognosed",
  "plannedDeparturePlatform": null,
  "additional": true
}

---

fixes #303

[derhuerst]

Thanks!