New buyer

curl --request POST \
  --url https://api.{gr4vy_id}.gr4vy.app/buyers \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "external_identifier": "412231123",
  "display_name": "John L."
}'

{
  "type": "buyer",
  "id": "fe26475d-ec3e-4884-9553-f7356683f7f9",
  "billing_details": {
    "type": "billing-details",
    "first_name": "John",
    "last_name": "Lunn",
    "email_address": "john@example.com",
    "phone_number": "+1234567890",
    "address": {
      "city": "London",
      "country": "GB",
      "postal_code": "789123",
      "state": "Greater London",
      "state_code": "GB-LND",
      "house_number_or_name": "10",
      "line1": "10 Oxford Street",
      "line2": "New Oxford Court",
      "organization": "Gr4vy"
    },
    "tax_id": {
      "value": "12345678931",
      "kind": "gb.vat"
    }
  },
  "created_at": "2013-07-16T19:23:00.000+00:00",
  "display_name": "John L.",
  "external_identifier": "user-789123",
  "merchant_account_id": "default",
  "updated_at": "2013-07-16T19:23:00.000+00:00",
  "account_number": "1234567"
}