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>
The inline declaration will most likely be taken as a suggestion and the compiler will optimize based on all factors, quick look-up returned this little excerp:
"The only time an inline virtual call can be inlined is when the compiler knows the "exact class" of the object which is the target of the virtual function call. This can happen only when the compiler has an actual object rather than a pointer or reference to an object. I.e., either with a local object, a global/static object, or a fully contained object inside a composite."

At this point, I'm not sure it matters, I'll implement the code review suggestion.