Add support for `Counting Products` in faceted search feature

[Vvirich]

Hi.

Currently there is no way to enable a product counting using FacetedSearch features.
Please add a withCountingProducts(Boolean isCountingProducts) method to io.sphere.sdk.search.model.FacetedSearchSearchModel like in FacetSearchModel. Because now in json response product count field always is null and there’s no convenient way to enable it using faceted search api.

Thanks.