Add GetRevision()

GetRevision returns a specific revision value for the key, or "KeyNotFound" if that message for this revision can't be found. See nats-io/nats.go#903

adr/ADR-8.md

@@ -143,6 +143,9 @@ The interface here is a guide of what should function in read-only mode.
 type RoKV interface {
 	// Get gets a key from the store
 	Get(key string) (Entry, error)
+
+	// GetRevision returns a specific revision value for the key
+	GetRevision(key string, revision uint64) (Entry, err error)
 
 	// History retrieves historic values for a key
 	History(ctx context.Context, key string) ([]Entry, error)