Add StateHistory property to PObject class

Introduces a new `StateHistory` property in the `PObject` class within `PObject.cs`. This property is decorated with the `[ForeignKey("ObjStateId")]` attribute and is of type `IEnumerable<PObjectStateHist>?`, allowing it to store a collection of `PObjectStateHist` objects. It is initialized to an empty array using `Array.Empty<PObjectStateHist>()`.

src/WorkFlow.Domain/Entities/PObject.cs

@@ -27,4 +27,7 @@ public class PObject
 
     [ForeignKey("ObjStateId")]
     public virtual PObjectState? State { get; set; }
+
+    [ForeignKey("ObjStateId")]
+    public virtual IEnumerable<PObjectStateHist>? StateHistory { get; set; } = Array.Empty<PObjectStateHist>();
 }