HPCC-11256 - Introduce new SDS node subscription mechanism

Allows clients to subscribe to existing SDS nodes, rather than
a dynamically matching xpath.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
HPCC-11256 Fix some issues following review
HPCC-11256 Minor changes following review

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Remove spurious use of simpleNotifyData
Merge pull request #5836 from jakesmith/hpcc-11256

HPCC-11256 - Introduce new SDS node subscription mechanism

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>