REST API. List products with parent/child relation (parent_sku is missing)

**Description**
parent_sku is missing for {{baseUrl}}/V1/products API response

**Expected behavior**
parent_sku is null in case if doesn't have any parent product relation (configurable products)

**Benefits**
easy way to build parent/child relations.

**Additional information**
I aware about {{baseUrl}}/V1/configurable-products/{sku}/children API. But it`s extra request each time in order to check if product has child product variants

Start by examining the /V1/products response and compare it with the /V1/configurable-products/{sku}/children endpoint to understand the existing parent/child information. The work is done when each product response includes parent_sku, using null when no parent product relation exists.

Written by the indexing model from the issue text.