HPCC-12660 Helper class for compressing and decompressing row pointers

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6708 from ghalliday/issue12660

HPCC-12660 Helper class for compressing and decompressing row pointers

Reviewed-By: Jake Smith <jake.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>