Save As Excel (#279)

* Fix dispose pattern usage in SaveAsWriterBase

* Add SaveAsExcel feature

This adds the save as excel function to the backend. To reduce large dependency and run on dotnet core now, this implementation use a raw excel writer (the SaveAsExcelFileStreamWriterHelper.cs) instrad of popular excel library, such as EPPlus or OpenXmlSdk.

* Fix can not open the generated excel file in google sheet

For the file name inside excel, google uses a case sensitive path while
Excel doesn't. This change fix the case, so that the file name matches
the one in x1/_rels/workbook.xml.rels

* Fix datetime doesn't recognized by google sheet

Google doesn't support cell type t="d" with ISO 8601 date. (From stackoverflow thread and testing), thus use the old way of excel datetime, which uses double to present datetime

* update to use xmlwriter

* Add basic unit tests for SaveAsExcelFileStreamWriterHelper

* refactor: simplify the public interface of the SaveAsExcelFileStreamWriterHelper

* update private fields names based on the name convention

* Add comments to classes of SaveAsExcel feature

* clean up SaveAsExcelFileStreamWriterHelper

- change SaveAsExcelFileStreamWriterHelper from public to internal
- remove the PenddingRowEndTag function from referenceManager
- change the SaveAsExcelFileStreamWriterHelper(stream) to default leaveOpen to false to match the normal behavior
- change the rowreference to use XmlConvert to convert int to string
- rename writeSetting to writerSetting and add private

* fix CI test error for SaveAsExcel

* remove ExporterException in SaveAsExcel

* fix lefe over CSV to Excel in the comments

* refactor to be consistent with JsonWriter and remove the comment

* remove commented out test

The test is too slow to run

* fix typo in comment

* refactor SaveAsExcel to the coding standard

* refactor rewrite the WriteStyle with XmlWriter

* Add licence header

* reverse mistakenly checked-in changes

* fix: left-over CSV in commets

* remove duplicate check

The check was done in the IncreaseColumnReference, but that check is too late in case of too many columns. All the addCell do the check at the begining now

* fix TimeSpan more than 24 hours

* fix AddRowMustBeCalledBeforeAddCellException test

This is  due to remove duplicate call to AssureColumnReference in  WriteAndIncreaseColumnReference

* fix: TimeSpan will write twice

* style: change retun in the switch to break

* Add bool format

* remove todo in comment

This provides extra safeguard in the cost of one memory access when null.

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

@@ -100,17 +100,19 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.DataStorage
         /// Disposes the instance by flushing and closing the file stream
         /// </summary>
         /// <param name="disposing"></param>
-        private void Dispose(bool disposing)
+        protected virtual void Dispose(bool disposing)
         {
-            if (disposed || !disposing)
-            {
-                disposed = true;
+            if (disposed)
                 return;
-            }
 
-            FileStream.Dispose();
+            if (disposing)
+            {
+                FileStream.Dispose();
+            }
+            disposed = true;
         }
-        public virtual void Dispose()
+
+        public void Dispose()
         {
             Dispose(true);
             GC.SuppressFinalize(this);