Add foundation

Toolkit.Foundation/IMediator.cs

@@ -0,0 +1,10 @@
+namespace Toolkit.Foundation;
+
+public interface IMediator
+{
+    Task<TResponse?> SendAsync<TResponse>(IRequest<TResponse> request,
+        CancellationToken cancellationToken = default);
+
+    Task<object?> SendAsync(object message, CancellationToken
+        cancellationToken = default);
+}