Refactor auditing interfaces to new Auditing namespace

Moved auditing interfaces to EnvelopeGenerator.Domain.Interfaces.Auditing. Updated entity classes to reference the new namespace. Added ICreationAuditable and IUpdateAuditable interfaces to improve code organization and clarify auditing responsibilities.

EnvelopeGenerator.Domain/Interfaces/Auditing/IHasChangedWhen.cs

@@ -0,0 +1,11 @@
+#if NETFRAMEWORK
+using System;
+#endif
+
+namespace EnvelopeGenerator.Domain.Interfaces.Auditing
+{
+    public interface IHasChangedWhen
+    {
+        DateTime? ChangedWhen { get; set; }
+    }
+}