Akma/login migrations (#1728)

In this PR, we make the appropriate backend service changes in order to enable the login migrations feature in the SQL migration extension.

Changes include:

updating the Microsoft.SqlServer.Migration.Login NuGet version to the latest version
adding a new request handler for StartLoginMigrations calls, which makes the appropriate calls to the login NuGet
adding ExtensionMethod helper to properly combine exception maps login migration nuget calls

Co-authored-by: Akshay Mata <akma@microsoft.com>

src/Microsoft.SqlTools.ServiceLayer/Migration/Utils/MigrationServiceHelper.cs

@@ -0,0 +1,17 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+
+namespace Microsoft.SqlTools.ServiceLayer.Migration.Utils
+{
+    internal static class MigrationServiceHelper
+    {
+        public static string FormatTimeSpan(TimeSpan ts)
+        {
+            return String.Format("{0:00}:{1:00}:{2:00}.{3:00}", ts.Hours, ts.Minutes, ts.Seconds, ts.Milliseconds / 10);
+        }
+    }
+}