Binds a customer to a cart and returns the updated cart. The cart is identified either by cart_id (the internal ID returned by every other Cart Builder endpoint, for a cart already known to Talkdesk) or by cart_token and cart_key together (the Shopify Storefront cart's own identity, for a cart the caller has never registered with Talkdesk before, e.g. one created directly via the merchant's storefront). At least one of these two identification modes must be provided; if both are present, cart_id takes precedence and cart_token/cart_key are ignored.