Add GetObjectKey method to MyEntity class

Introduced the GetObjectKey method to MyEntity, returning the string representation of the entity's Id. This may be used for scenarios requiring a unique key in string format.

framework/test/Volo.Abp.Ddd.Tests/Volo/Abp/Domain/Entities/EntityHelper_Tests.cs

@@ -55,6 +55,11 @@ public class EntityHelper_Tests
         {
             return new object[] { Id };
         }
+
+        public string GetObjectKey()
+        {
+            return Id.ToString();
+        }
     }
 
     private class MyEntityDisablesIdGeneration : Entity<Guid>