Added a separate resolver for returning category products count

[rogyar]

Description (*)

The product_count field of the category query now returns a value from the corresponding field of catalog_category_entity. Using this approach brings additional issues since the value in the database sometimes does not reflect the real products count. Moreover, if we request products of this category, the GraphQL query returns only products that are saleable (visible and in stock). So product_count value will show the different result if some products of the category are out of stock or not visible.

Fixed Issues (if relevant)

1. Incorrect returned values of product_count #214 : Incorrect returned values of product_count
2. Configurable Products counted but not retrieved in GraphQL (works with REST endpoint) #238 : Configurable Products counted but not retrieved in GraphQL (works with REST endpoint)

Manual testing scenarios (*)

1. Add some products to a category and make some of them out of stock or not visible individually
2. Use a similar query to retrieve the category and products:

{
  category(id: 1158) {
    product_count
    products {
      items {
        name
      }
      total_count
    }
  }
}

3. The value in product_count field should corresponds to the total number of fetched products
4. The value in total_count field should equal the value in products_count

[paliarush]

Ideally we should have some test cases for tricky scenarios, like:

• show products out of stock
• configurables with out of stock children
• products available in one store, but not available in the other

[naydav]

Corresponding ticket has been created
#255