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>
Still sounds a bit strange to me.  When I see an inline method in an interface I assume that its only a convenience function that no one will ever override.  I'm not even sure what the compiler will do if the function is both inline and virtual. @ghalliday any thoughts?