Add debug notifications

This adds a test notif only available in debug mode + endpoint `/Notifications/GenerateJunk` that generates x amount of notifications.

BTCPayServer/Services/Notifications/Blobs/JunkNotification.cs

@@ -0,0 +1,19 @@
+#if DEBUG
+using BTCPayServer.Models.NotificationViewModels;
+
+namespace BTCPayServer.Services.Notifications.Blobs
+{
+    internal class JunkNotification
+    {
+        internal class Handler : NotificationHandler<JunkNotification>
+        {
+            public override string NotificationType => "junk";
+
+            protected override void FillViewModel(JunkNotification notification, NotificationViewModel vm)
+            {
+                vm.Body = $"All your junk r belong to us!";
+            }
+        }
+    }
+}
+#endif