Added validation and fixed errors

SchengenVisaApi/ApplicationLayer/InfrastructureServicesInterfaces/IUserIdProvider.cs

@@ -0,0 +1,8 @@
+namespace ApplicationLayer.InfrastructureServicesInterfaces
+{
+    public interface IUserIdProvider
+    {
+        /// Returns identifier of authenticated user who sent the request
+        Guid GetUserId();
+    }
+}