Patch product by gateway account ID and product external ID

Example request

{
  "amount_hint": "Enter an amount in multiples of £2 for the number of permits required",
  "description": "New description of the product",
  "name": "name-for-product",
  "price": 1240,
  "reference_enabled": true,
  "reference_hint": "This can be found on your letter",
  "reference_label": "Amount for your licence 2"
}