Add an efficient way to fetch space labels in v2 REST APIs

Suggestion Summary

Currently with the v2 APIs, it is only possible to retrieve labels for spaces one space at a time using /wiki/api/v2/spaces/{id}/labels endpoint. To retrieve labels for multiple spaces, multiple API calls will need to be made which is inefficient when dealing with a large number of spaces.

A solution should be implemented to allow clients to efficiently retrieve space labels in the v2 APIs.