Export to Markdown Support (#1705)

* Adding file writer for Markdown tables. No testing yet.

* Unit tests for the markdown writer

* Wiring up the factory and and request types

* Wiring up changes for Markdown serialization in serialization service

* Couple last minute tweaks

* Changes as per PR comments

* Revert temp testing code. 🙈

* Fluent assertions in SerializationServiceTests.cs

Co-authored-by: Ben Russell <russellben@microsoft.com>

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

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Text;
 using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
 using Microsoft.SqlTools.Utility;
 
@@ -106,6 +107,37 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.DataStorage
             FileStream.Flush();
         }
 
+        /// <summary>
+        /// Attempts to parse the provided <paramref name="encoding"/> and return an encoding that
+        /// matches the encoding name or codepage number.
+        /// </summary>
+        /// <param name="encoding">Encoding name or codepage number to parse.</param>
+        /// <param name="fallbackEncoding">
+        /// Encoding to return if no encoding of provided name/codepage number exists.
+        /// </param>
+        /// <returns>
+        /// Desired encoding object or the <paramref name="fallbackEncoding"/> if the desired
+        /// encoding could not be found.
+        /// </returns>
+        protected static Encoding ParseEncoding(string encoding, Encoding fallbackEncoding)
+        {
+            // If the encoding is a number, we try to look up a codepage encoding using the
+            // parsed number as a codepage. If it is not a number, attempt to look up an
+            // encoding with the provided encoding name. If getting the encoding fails in
+            // either case, we will return the fallback encoding.
+            Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+            try
+            {
+                return int.TryParse(encoding, out int codePage)
+                    ? Encoding.GetEncoding(codePage)
+                    : Encoding.GetEncoding(encoding);
+            }
+            catch
+            {
+                return fallbackEncoding;
+            }
+        }
+
         #region IDisposable Implementation
 
         private bool disposed;