Expose root_storage_statistics in REST API in groups endpoint

Currently when requesting GET /api/v4/groups?statistics=true, the following structure can be found:

statistics": {
 "commit_count": 1,
 "storage_size": 3281,
 "repository_size": 3281,
 "wiki_size": 0,
 "lfs_objects_size": 0,
 "job_artifacts_size": 0,
 "pipeline_artifacts_size": 0,
 "packages_size": 0,
 "snippets_size": 0,
 "uploads_size": 0
 },

Similar to the GraphQL API, it should also be possible to get the root_storage_statistics on group level.