users: Send user creation events when sending DMs.

We now send user creation events to recipient users
when sending DMs if recipients gain access to either
sender or other pariticpating users in the DM.

zerver/lib/message.py

@@ -209,6 +209,7 @@ class SendMessageRequest:
     service_queue_events: Optional[Dict[str, List[Dict[str, Any]]]] = None
     disable_external_notifications: bool = False
     automatic_new_visibility_policy: Optional[int] = None
+    recipients_for_user_creation_events: Optional[Dict[UserProfile, Set[int]]] = None
 
 
 # We won't try to fetch more unread message IDs from the database than