feat(codegen): Adds AfterDeserialization support (#615)

Added the ability to execute arbitrary code after deserialization.

Example, let's say you want to delete an item after you deserialize it:
```cs
        [AfterDeserialization]
        private void OnAfterDeserialization()
        {
            Delete();
        }
```

Generates this:
```cs
        public override void Deserialize(IGenericReader reader)
        {
            base.Deserialize(reader);

            var version = reader.ReadEncodedInt();

            Timer.DelayCall(OnAfterDeserialization);
        }
```

Projects/Server/Serialization/AfterDeserialization.cs

@@ -0,0 +1,28 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2021 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: AfterDeserialization.cs                                         *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using System;
+
+namespace Server
+{
+    /// <summary>
+    /// Hints to the source generator that this method should be executed after deserializing the object.
+    /// Method must have no parameters and return void.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method)]
+    public class AfterDeserializationAttribute : Attribute
+    {
+    }
+}