Allow to override prices at checkout

[frankie567]

trafficstars

For advanced use cases, it has become clear that some merchants need to be able to set a completely dynamic pricing set when creating the Checkout Session.

Basically, we would allow to pass a set of prices on the Checkout session. Those prices would be created in database like the other prices, but should be marked specifically so they don't appear as the "true" prices of the product.

Proposed API:

{
    "products": ["PRODUCT_ID_1", "PRODUCT_ID_2"],
    "prices": {
        "PRODUCT_ID_1": [
            {
                "type": "fixed",
                "amount": 1000,
                "currency": "usd",
            }
        ]
    }
}