feat(flights): add FlightStop structure

[0x9ef]

Duffel has "stops" (can be nullable) field (list) in Flight segment with the next Duffel's description:

Additional segment-specific information about the stops, if any, included in the segment

Has the next body:

  "stops": [
    {
      "id": "sto_00009htYpSCXrwaB9Dn456",
      "duration": "PT02H26M",
      "departing_at": "2020-06-13T16:38:02",
      "airport": {
        "type": "airport",
        "time_zone": "Europe/London",
        "name": "Heathrow",
        "longitude": -141.951519,
        "latitude": 64.068865,
        "id": "arp_lhr_gb",
        "icao_code": "EGLL",
        "iata_country_code": "GB",
        "iata_code": "LHR",
        "iata_city_code": "LON",
        "city_name": "London",
        "city": {
          "name": "London",
          "id": "cit_lon_gb",
          "iata_country_code": "GB",
          "iata_code": "LON"
        },
        "airports": [
          {
            "time_zone": "Europe/London",
            "name": "Heathrow",
            "longitude": -141.951519,
            "latitude": 64.068865,
            "id": "arp_lhr_gb",
            "icao_code": "EGLL",
            "iata_country_code": "GB",
            "iata_code": "LHR",
            "iata_city_code": "LON",
            "city_name": "London",
            "city": {
              "name": "London",
              "id": "cit_lon_gb",
              "iata_country_code": "GB",
              "iata_code": "LON"
            }
          }
        ]
      }
    }
  ]