HPCC-10674 ECL support for writing JSON files

Test case will improve with the upcoming addition of reading json files.. as I can then read and output what has first been written.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
I don't think this interface needs to be in this file (it isn't called from generated code), and should probably be in thorxmlwrite.hpp.
It is subtly different as well.  The IXmlWriter interface could model something that is dynamically spilled to disk, whilst this interface implies it is all held in memory.  I'm not sure that is very significant though.