[API] Implement Endpoints for Subscription Analytics

Implement the three analytics endpoints defined in the design document: GET /v1/stats/features/top-subscribed, GET /v1/stats/features/{feature_id}/subscriber-count, and the authenticated GET /v1/users/me/saved-searches/{search_id}/stats. The spanneradapters layer will need new methods to query the Analytics_SavedSearch_Feature_Map table to ensure data is both fast and accurate.