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'm not sure if I would have added a new TAKxxx or added a flag to the helper of TAKxmlwrite.  Either is reasonable.