Add Version property to OrchestrationContext class

Author: AnatoliB

src/DurableSDK/OrchestrationContext.cs

@@ -36,5 +36,19 @@ public class OrchestrationContext
         internal OrchestrationActionCollector OrchestrationActionCollector { get; } = new OrchestrationActionCollector();
 
         internal object CustomStatus { get; set; }
+
+        public string Version 
+        {
+            get 
+            {
+                if (History == null)
+                {
+                    return null;
+                }
+                
+                var executionStartedEvent = Array.Find(History, e => e.EventType == HistoryEventType.ExecutionStarted);
+                return executionStartedEvent?.Version;
+            }
+        }
     }
 }