feat: add inital in memory dispatcher

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

src/request.tests/_fixtures/ConstrainedScalarHandler.cs

@@ -0,0 +1,13 @@
+// Copyright (c) The Geekeey Authors
+// SPDX-License-Identifier: EUPL-1.2
+
+namespace Geekeey.Request.Tests;
+
+public class ConstrainedScalarHandler<TRequest> : IScalarRequestHandler<TRequest, string>
+	where TRequest : ConstrainedScalarRequest
+{
+	public Task<string> HandleAsync(TRequest request, CancellationToken cancellationToken)
+	{
+		return Task.FromResult($"{request.Value}-Constrained");
+	}
+}