List price codes

Returns every price code configured for a product, ordered by `price_code`. A price code lets one product serve a different price to a group of customers. Each price code maps a code to a price, and a product can carry many of them. Ordergroove resolves a code for each order from the customer or from the shipping address, depending on how the merchant's price driver is configured, and charges the price mapped to that code. When the product has no price code matching the resolved value, the product's base `price` applies. By convention, codes resolved from the customer are prefixed `Cust_` and codes resolved from the shipping address are prefixed `Ship_`, but a code is matched exactly and any string is accepted. These are the same price codes the XML product feed writes, so a catalog can be priced by code through either path. Price codes are scoped to the merchant that owns the product, so a request for another merchant's product returns `404`, not `403`. Both Application API Scope and Storefront API Scope credentials can read price codes; only Application API Scope can change them.