Adding StateMachine for Session serialization

- adding multiple classes and interfaces for Session Serialization and recovery after restart

TelegramBotBase/Attributes/SaveState.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace TelegramBotBase.Attributes
+{
+    /// <summary>
+    /// Declares that the field or property should be save and recovered an restart.
+    /// </summary>
+    public class SaveState : Attribute
+    {
+        public String Key { get; set; }
+
+    }
+}