feat: NotificationService interface

at_client/src/main/java/org/atsign/client/api/notification/NotificationService.java

@@ -0,0 +1,23 @@
+package org.atsign.client.api.notification;
+
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+
+import org.atsign.client.api.AtClient;
+import org.atsign.client.api.AtEvents;
+import org.atsign.client.api.impl.clients.NotificationServiceImpl;
+
+public interface NotificationService {
+
+    public static NotificationService create(AtClient atClient) {
+        return new NotificationServiceImpl(atClient);
+    }
+
+    CompletableFuture<Object> subscribe(String regex, boolean shouldDecrypt); // TODO Replace "Object" with Stream<AtNotification> or similar. See: https://github.com/atsign-foundation/at_client_sdk/blob/99707459494594fbdc3f0873769eeca29c6a3124/packages/at_client/lib/src/service/notification_service.dart#L14   
+    CompletableFuture<NotificationResult> notify(NotificationParams params);
+    CompletableFuture<NotificationResult> getStatus(String notificationId);
+    CompletableFuture<List<AtNotification>> notifyList();
+    CompletableFuture<NotificationResult> notifyDelete(String notificationId);
+    CompletableFuture<NotificationResult> notifyDeleteAll();
+
+}