HPCC-12062 Add FromJSON function to ECL

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
HPCC-12062 Update based on review

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #6329 from afishbeck/fromJSON

HPCC-12062 Add FromJSON function to ECL

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>