HPCC-12755 Rationalize secure workunit interface

Another step towards being able to provide plugin workunit functionality.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-12755 Rationalize secure workunit interface

Minor changes following code review

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6769 from richardkchapman/cassandra-wu

HPCC-12755 Rationalize secure workunit interface

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>