Add support for search and including project labels within Group Labels API

Summary

Currently in Group context, we use private API (eg; /-/labels.json?include_ancestor_groups=true) to fetch labels which includes all the labels in the response. This is the case for labels dropdown in sidebar for Issues, MRs and Epics.

This has also been a performance problem for a while as now we have over 1000 labels for gitlab-org group in GitLab.com. As we're migrating labels dropdown to labels_select_vue across GitLab, it would be sensible to make the dropdown use Public API and perform searching of labels an async action (we're already doing it for Epics and Milestones dropdown currently).

Currently, both Group Labels API & Project Labels API lack support for search param. Additionally, Group Labels API doesn't support including project labels (via include_project_labels param for example), so we need to update both these endpoints to have search support and Group Labels API specifically to have project labels fetching support.