Hpcc 11922 new json support

Added JSON support 
Tony's IXmlWriterExt changes cherry-picked
 - https://github.com/afishbeck/HPCC-Platform/commit/6d217720cf470d9f7773a8193c46f56bf0bbd439
 - from  branch afishbeck:jsonFileWrite

Integrated IXmlWriterExt changes
Signed-off-by: rpastrana <rodrigo.pastrana@lexisnexis.com>
I think this breaks the model of this interface and should probably be done by the caller instead.  Plus this would be the only thing that forces users and implementations of this interface to load "jstring.hpp"... not sure if any of the core libraries had avoided this so far or not.