BA-639 Add set articles to KlarnaKP Pay function

[gijsmin]

This code adds the possibility to sent an optional list of articles together with the KlarnaKP pay request.

[gijsmin]

It looks like this needs some more work from the Buckaroo side, since sending a list of articles with the pay request results in incomplete requests from Buckaroo to Klarna:

"Services": {
    "ServiceList": [
      {
        "Action": "Pay",
        "Name": "klarnakp",
        "Version": 1,
        "Parameters": [
          {
            "Name": "ReservationNumber",
            "Value": "fe11d0f9-dce4-4188-9ad0-9e32e19d0038"
          },
          {
            "Name": "Identifier",
            "Value": "H131365"
          },
          {
            "Name": "Type",
            "Value": "General"
          },
          {
            "Name": "Price",
            "Value": 139.99
          },
          {
            "Name": "Quantity",
            "Value": 1
          },
          {
            "Name": "VatPercentage",
            "Value": 19
          },
       ]
}

[Buckaroo-Rene]

Good morning @gijsmin,

Thank you very much for your PR. I'll have one of our developers review it to determine if any additional adjustments are needed or if it's ready to be merged.