Add missing file

BTCPayServer/Extensions/ServiceCollectionExtensions.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using BTCPayServer.Hosting;
+
+namespace Microsoft.Extensions.DependencyInjection
+{
+    public static class ServiceCollectionExtensions
+    {
+        public static IServiceCollection AddStartupTask<T>(this IServiceCollection services)
+            where T : class, IStartupTask
+            => services.AddTransient<IStartupTask, T>();
+    }
+}