HPCC-10219 Share common base with all join and other join helpers

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5630 from ghalliday/issue10219

HPCC-10219 Share common base with all join and other join helpers

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>