fix: accidentally removed codes

Author: insthync

SimpleNetworkManager.NET/Network/BaseClientConnection.cs

@@ -66,6 +66,11 @@ public void OnDisconnected()
         /// </summary>
         public abstract UniTask SendMessageAsync(BaseMessage message);
 
+        /// <summary>
+        /// Disconnects the client gracefully
+        /// </summary>
+        public abstract UniTask DisconnectAsync();
+
         /// <summary>
         /// Sends a serialization error message to the client
         /// </summary>