UBERF-7513: Improve notifications model to allow external notifications channels (#6037)

Signed-off-by: Kristina Fefelova <kristin.fefelova@gmail.com>

server-plugins/activity/src/index.ts

@@ -15,6 +15,7 @@
 
 import { Plugin, Resource, plugin } from '@hcengineering/platform'
 import type { TriggerFunc } from '@hcengineering/server-core'
+import { NotificationContentProvider } from '@hcengineering/server-notification'
 
 export * from './types'
 export * from './utils'
@@ -33,5 +34,8 @@ export default plugin(serverActivityId, {
     OnDocRemoved: '' as Resource<TriggerFunc>,
     OnReactionChanged: '' as Resource<TriggerFunc>,
     ReferenceTrigger: '' as Resource<TriggerFunc>
+  },
+  function: {
+    ReactionNotificationContentProvider: '' as Resource<NotificationContentProvider>
   }
 })