Change checkout custom data type to key value pairs

[JulianBerger]

The custom data needs to be key value pairs as lemon squeezy shows in their docs: https://docs.lemonsqueezy.com/help/checkout/passing-custom-data

{
  "meta": {
    "event_name": "order_created",
    "custom_data": {
      "user_id": "123"
    }
  },
  ...
}

[JoshZA]

👍 This is needed

[NuroDev]

Closing as this has already been fixed as part of #20