feat: Add assignee last seen to conversations (#3069)

- Adds assignee last seen attribute to conversations

app/views/api/v1/accounts/conversations/messages/index.json.jbuilder

@@ -4,6 +4,7 @@ json.meta do
   json.contact @conversation.contact.push_event_data
   json.assignee @conversation.assignee.push_event_data if @conversation.assignee.present?
   json.agent_last_seen_at @conversation.agent_last_seen_at
+  json.assignee_last_seen_at @conversation.assignee_last_seen_at
 end
 
 json.payload do