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 think I was looking ahead to simplifying some aspect of spray/despray, but I'll simplify for now and adjust as I or Atilla actually work on that bit.