Unl change announcement to connected users. (#224)

Introduced json and bson messages for unl list announcement.
When the unl set is modified send a json or bson unl list message (according to the user protocol) to all the connected users.

src/usr/usr.cpp

@@ -417,4 +417,24 @@ namespace usr
         }
     }
 
+    /**
+     * Send unl list to all the connected users.
+     * @param unl_list Set of unl pubkeys.
+    */
+    void announce_unl_list(const std::set<std::string> &unl_list)
+    {
+        std::scoped_lock<std::mutex> lock(ctx.users_mutex);
+
+        for (const auto &user : ctx.users)
+        {
+            const usr::connected_user &connected_user = user.second;
+            msg::usrmsg::usrmsg_parser parser(connected_user.protocol);
+
+            std::vector<uint8_t> msg;
+            parser.create_unl_list_container(msg, unl_list);
+
+            connected_user.session.send(msg);
+        }
+    }
+
 } // namespace usr