feat: add inital in memory dispatcher

Add a simple in memory dispatcher for scalar requests and stream request.

src/request.tests/_fixtures/FailingScalarHandler.cs

@@ -0,0 +1,12 @@
+// Copyright (c) The Geekeey Authors
+// SPDX-License-Identifier: EUPL-1.2
+
+namespace Geekeey.Request.Tests;
+
+public class FailingScalarHandler : IScalarRequestHandler<FailingScalarRequest, string>
+{
+	public Task<string> HandleAsync(FailingScalarRequest request, CancellationToken cancellationToken)
+	{
+		throw new InvalidOperationException("Handler failed");
+	}
+}