To provide an endpoint that swatch contracts can use to map a SKU to the swatch product IDs. 

• swatch-subscriptions-sync Internal API changes (for inter-service integration)
• GET /internal/offerings/{sku}/product_tags
• This endpoint will allow any service to lookup the swatch product(s) associated with a given SKU. (This lookup will use the offering information already stored in the database). We already store in the offering table the `product_name`, and map the `product_name` to a swatch `product_id` via info in `tag_profile.yaml`. Navigate this mapping the other direction: given a SKU, what tags does it map to via the recorded `product_name`.
• Define new API src/main/spec/internal-subscriptions-sync-api-spec.yaml

QE: given this is intended to support mapping a contract, testing of this functionality should be covered by SWATCH-805 instead (this also avoids coupling QE tests to this implementation detail).