Support for choosing which BackLinks get's retrived when quering database

[Hossein-Gholami]

In case of trying to query a collection that has Link to other collections, it is possible to fetch links on-demand with two methods: fetch_all_links and fetch_link that in the latter, the Link field could be selected. In case of BackLink there's no way one could fetch it on-demand, but it would be great if it was possible to choose the BackLink field that needs to be fetched. This feature is somehow related to the possibility of fetching BackLinks on-demand like Links.