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>
But even if we keep the more efficient way of saying its a numeric string, we may at least need a new method so old generated code doesn't call this one with the wrong parameters.. or we could do as Richard suggests we could use and extended interface and do a dynamic_cast if necessary to check for / get to it.