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/SerializationService.cs

@@ -243,6 +243,9 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
                             SaveRequestParams = CreateCsvRequestParams()
                         };
                         break;
+                    case "markdown":
+                        factory = new SaveAsMarkdownFileStreamFactory(CreateMarkdownRequestParams());
+                        break;
                     case "xml":
                         factory = new SaveAsXmlFileStreamFactory()
                         {
@@ -304,6 +307,18 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
                 MaxCharsToStore = this.requestParams.MaxCharsToStore
             };
         }
+
+        private SaveResultsAsMarkdownRequestParams CreateMarkdownRequestParams() =>
+            new SaveResultsAsMarkdownRequestParams
+            {
+                FilePath = this.requestParams.FilePath,
+                BatchIndex = 0,
+                ResultSetIndex = 0,
+                IncludeHeaders = this.requestParams.IncludeHeaders,
+                LineSeparator = this.requestParams.LineSeparator,
+                Encoding = this.requestParams.Encoding,
+            };
+
         private SaveResultsAsXmlRequestParams CreateXmlRequestParams()
         {
             return new SaveResultsAsXmlRequestParams