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>
This method is definitely a prime candidate for being in-lined.... I wonder if the compiler is sophisticated enough to automatically inline the method without the explicit inline method decoration.... 
I'll go ahead and explicitly add the keyword in order to keep this helpful method implementation so that none of the IXmlWriterExt implementations need to concern themselves, unless of course they need to overwrite it...