[spanneradapter] Add Adapter Method to Get Event Metadata #1877
Description
In lib/gcpspanner/spanneradapters, create a new adapter method (e.g., GetEventMetadata) that the paginated diff endpoint will call. This method will take an EventId and return the necessary metadata from the SavedSearchNotificationEvents table (GcsBlobPath, DiffKind, DiffSchemaVersion) required by the API to select the correct parser and find the GCS blob.