Added plugin events

src/HopFrame.Core/Callbacks/HopCallbackHandler.cs

@@ -0,0 +1,7 @@
+namespace HopFrame.Core.Callbacks;
+
+public readonly struct HopCallbackHandler(string eventType, Func<object, IServiceProvider, Task> handler) {
+    public Guid Id { get; } = Guid.CreateVersion7();
+    public Func<object, IServiceProvider, Task> Handler { get; } = handler;
+    public string EventType { get; } = eventType;
+}