Add configurable layers in manifest events

Signed-off-by: Andrew Leung <anwleung@gmail.com>

cmd/registry/config-cache.yml

@@ -29,6 +29,8 @@ redis:
   readtimeout: 10ms
   writetimeout: 10ms
 notifications:
+    events:
+        manifestlayers: true
     endpoints:
         - name: local-8082
           url: http://localhost:5003/callback

cmd/registry/config-dev.yml

@@ -47,6 +47,8 @@ redis:
   readtimeout: 10ms
   writetimeout: 10ms
 notifications:
+    events:
+        manifestlayers: true
     endpoints:
         - name: local-5003
           url: http://localhost:5003/callback