Campaign products accept ALL_FEE_FREE

products on POST /campaigns and PUT /campaigns/{id} now accepts the special value ALL_FEE_FREE. We replace it with the IDs of every product in your catalog that carries no fee at the time of the call. Products that launch later are not added to the campaign. Use auto_add_product_rule to keep adding new fee-free gift cards automatically. To include a product regardless of its fees, add its ID alongside the value: ["ALL_FEE_FREE", "KV934TZ93NQM"].

GET /products takes a new id filter that accepts the same values, so GET /products?id=ALL_FEE_FREE shows the products that don't carry a fee.

ALL_FEE_FREE is never returned on read.