WIP to allow mediter handlers to be subscribed to

Toolkit.Foundation/HandlerProvider.cs

@@ -0,0 +1,26 @@
+namespace Toolkit.Foundation;
+
+public class HandlerProvider(SubscriptionCollection subscriptions) : 
+    IHandlerProvider
+{
+    public IEnumerable<object?> Get(Type type,
+        object key)
+    {
+        string subscriptionKey = $"{(key is not null ? $"{key}:" : "")}{type}";
+        if (subscriptions.TryGetValue(subscriptionKey, out List<WeakReference>? subscribers))
+        {
+            foreach (WeakReference weakRef in subscribers.ToArray())
+            {
+                object? target = weakRef.Target;
+                if (target != null)
+                {
+                    yield return target;
+                }
+                else
+                {
+                    subscribers.Remove(weakRef);
+                }
+            }
+        }
+    }
+}