Add foundation

Toolkit.Foundation/Subscriber.cs

@@ -0,0 +1,11 @@
+namespace Toolkit.Foundation;
+
+public class Subscriber(ISubscriptionManager subscriptionManager) : 
+    ISubscriber
+{
+    public void Remove(object subscriber) => 
+        subscriptionManager.Remove(subscriber);
+
+    public void Add(object subscriber) => 
+        subscriptionManager.Add(subscriber);
+}