Add a `shared_projects` endpoint for groups
Add a new /groups/gitlab-org/-/shared_projects.json
endpoint that serves GroupDescendantSerializer
JSON, the same JSON as the /groups/gitlab-org/-/children.json
endpoint.
The endpoint needs to support the filter
and the sort
query params.
This will be the backend for https://gitlab.com/gitlab-org/gitlab-ce/issues/44005.