feat: add pub/sub support to dispatcher

Notifications are broadcast to zero or more handlers with configurable
publishing strategies (sequential or parallel) and pipeline behavior
support.

src/request.dispatcher.tests/_fixtures/OtherNotificationPublisher.cs

@@ -0,0 +1,17 @@
+// Copyright (c) The Geekeey Authors
+// SPDX-License-Identifier: EUPL-1.2
+
+namespace Geekeey.Request.Dispatcher.Tests;
+
+public class OtherNotificationPublisher(NotificationTestTracker tracker) : INotificationPublisher
+{
+	public async Task PublishAsync<TNotification>(IEnumerable<INotificationHandler<TNotification>> handlers, TNotification notification, CancellationToken cancellationToken) where TNotification : INotification
+	{
+		tracker.Log.Add("OtherPublisher");
+
+		foreach (var handler in handlers)
+		{
+			await handler.HandleAsync(notification, cancellationToken);
+		}
+	}
+}