[spanneradapter] Create Data Adapter for Ingestion Worker

[jcscottiii]

In lib/gcpspanner/spanneradapters, create a new adapter to expose the high-level methods required by the EventProducerWorker (e.g., AcquireWorkerLock, UpdateState, LogEvent). This will abstract the underlying mapper logic for the SavedSearchState and SavedSearchNotificationEvents tables.