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>
As it turns out, XML and JSON represent numerics slightly different (I don't remember the exact difference, quoted vs un-quoted, or something to that degree). Tony and I discussed the proper handling of numerics and determined it wasn't always worthwhile to convert string representations of a numerical value, to its native representation, to then serialize it to XML, or JSON. It seemed advantageous to provide the value as a string, and notify the IXmlWriter implementation and let the implementation decide how to serialize the string representation.