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 don't like this isNumeric parameter. What is it for? How/when should the caller be setting it true? Should we instead be creating a new IJsonWriter interface that extends IXmlWriter with a new method?

@ghalliday What do you think?