HPCC-12594 Allow DATASET(value) in a record to add a dataset field

By default using a dataset in a record implicitly expands all the fields out

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6672 from ghalliday/issue12594

HPCC-12594 Allow DATASET(value) in a record to add a dataset field

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>