Fix non-functional attributes in datasource gitlab_group_subgroups

Summary

This change fixes the filtering options available when looking up subgroups within a GitLab group. Previously, only a limited set of filters could be applied; now users can also filter by additional criteria such as search terms, sort order, access level, ownership, and whether to include custom attributes or group statistics. Each filter is only applied if the user has actually provided a value for it, avoiding unintended defaults.

Fixes #6838 (closed)