Added Validation logics

Toolkit.Foundation/IValidation.cs

@@ -0,0 +1,20 @@
+using System.ComponentModel;
+using System.Linq.Expressions;
+
+namespace Toolkit.Foundation;
+
+public interface IValidation : 
+    INotifyPropertyChanged
+{
+    ValidationErrorCollection Errors { get; }
+
+    bool HasErrors { get; }
+
+    void Add<TProperty>(Expression<Func<TProperty>> property,
+        ValidationRule[] rules,
+        ValidationTrigger trigger = ValidationTrigger.Deferred);
+
+    bool Validate();
+
+    bool Validate(string name);
+}