Returns the customers assignment for a list of customers and promocodes

Request body: pass an object where each key is a customerId and each value is the promocodeId assigned to that customer.
Example: { "customer_123": "PROMOCODE_A", "customer_456": "PROMOCODE_B" }

Only customers that have an assignment for the requested promocodeId are returned, so the response map may contain fewer entries than the request — a missing key means no assignment was found for that customer/promocode pair.