Checkout: Ability to override pre-selected product in `products`

[birkjernstrom]

Context: https://x.com/d4m1n/status/1893619144043475021

Customers experienced with using LS before expect the ability to add ?product_id=<>to query params in order to override the default, pre-selected, product in the array.

Today, we default to index 0, as the feature was designed to be programmatically used with our Checkout API (allowing any order given) or considered that a Checkout Link would prefer to have a static & specific order.

However, in case of the latter, some customers seem to prefer using a Checkout Link for ease, but with the ability to change the query param here and there to change the pre-selected product.