HPCC-10507 Add new MAXLENGTH option onto INDEX and BUILD

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5765 from ghalliday/issue10507

HPCC-10507 Add new MAXLENGTH option onto INDEX and BUILD

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>