Support additional LabelsFinder parameters for group labels

In some situations (listing labels for epics) we want to list only group ancestor or descendant labels. This MR passws required params to LabelsFinder and adds support for finding descendant labels.

Also set_issuables_index method now loads project labels only if @Project is set (which is not used for epic group labels).

This code is not used anywhere yet (so no changelog is included), but it will be used in the following MR: gitlab-ee!4773

EE version of the patch: gitlab-ee!4780