BE: Exclude child items from available options in work items add child item form

Similar to Parents widget( #460738), exclude child items while adding work items using Child Items widget.

While adding pagination, it was observed that the logic to filter the children is on the frontend. The issue with keeping the logic in frontend is, it will only have the work item ids which are fetched in the query. So, to fix it, an additional query call is added here to get all of the records. To avoid such issue, it is better to move the logic to backend and filter the child records with help of a parameter. Check this discussion thread for a proposed solution in the Parent widget's issue.