Add foundation

Toolkit.Foundation/IHandler.cs

@@ -0,0 +1,17 @@
+namespace Toolkit.Foundation;
+
+public interface IHandler;
+
+public interface IHandler<in TRequest, TResponse> :
+    IHandler
+    where TRequest :
+    IRequest<TResponse>
+{
+    Task<TResponse> Handle(TRequest args,
+        CancellationToken cancellationToken);
+}
+
+public interface IHandler<in TRequest> :
+    IHandler<TRequest, Unit>
+    where TRequest :
+    IRequest<Unit>;