[spanneradapter] Add Adapter Method for RSS "Matchmaking" #1876
Description
In the lib/gcpspanner/spanneradapters layer, create a new adapter method for the RSS endpoint's complex data needs. This method will need to find all SavedSearchSubscriptions for a given ChannelId and then dynamically build and execute the UNION ALL query against the SavedSearchNotificationEvents table to fetch the 50 most recent, correctly-filtered events.