Save as XML feature added (#684)

Similar approach used like Save as JSON or Save as CSV.

src/Microsoft.SqlTools.ServiceLayer/QueryExecution/DataStorage/SaveAsXmlFileStreamFactory.cs

@@ -0,0 +1,71 @@
+// 
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.IO;
+using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
+using Microsoft.SqlTools.ServiceLayer.SqlContext;
+using Microsoft.SqlTools.ServiceLayer.Utility;
+
+namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.DataStorage
+{
+    public class SaveAsXmlFileStreamFactory : IFileStreamFactory
+    {
+
+        #region Properties
+
+        /// <summary>
+        /// Settings for query execution
+        /// </summary>
+        public QueryExecutionSettings QueryExecutionSettings { get; set; }
+
+        /// <summary>
+        /// Parameters for the save as XML request
+        /// </summary>
+        public SaveResultsAsXmlRequestParams SaveRequestParams { get; set; }
+
+        #endregion
+
+        /// <summary>
+        /// File names are not meant to be created with this factory.
+        /// </summary>
+        /// <exception cref="NotImplementedException">Thrown all times</exception>
+        [Obsolete]
+        public string CreateFile()
+        {
+            throw new InvalidOperationException();
+        }
+
+        /// <summary>
+        /// Returns a new service buffer reader for reading results back in from the temporary buffer files
+        /// </summary>
+        /// <param name="fileName">Path to the temp buffer file</param>
+        /// <returns>Stream reader</returns>
+        public IFileStreamReader GetReader(string fileName)
+        {
+            return new ServiceBufferFileStreamReader(new FileStream(fileName, FileMode.Open, FileAccess.Read), QueryExecutionSettings);
+        }
+
+        /// <summary>
+        /// Returns a new XML writer for writing results to a XML file
+        /// </summary>
+        /// <param name="fileName">Path to the XML output file</param>
+        /// <returns>Stream writer</returns>
+        public IFileStreamWriter GetWriter(string fileName)
+        {
+            return new SaveAsXmlFileStreamWriter(new FileStream(fileName, FileMode.Create, FileAccess.ReadWrite), SaveRequestParams);
+        }
+
+        /// <summary>
+        /// Safely deletes the file
+        /// </summary>
+        /// <param name="fileName">Path to the file to delete</param>
+        public void DisposeFile(string fileName)
+        {
+            FileUtilities.SafeFileDelete(fileName);
+        }
+
+    }
+}