HPCC-8845 Support XMLNS for workunit and roxie xml outputs

Rebased.  New pull request for candidate 5.0.0 branch.

Previous pull request, with code review comments: https://github.com/hpcc-systems/HPCC-Platform/pull/5700

Add a new "XMLNS" option to ECL output for declaring the URI for xml namespace prefixes.

Example:

OUTPUT(ds, NAMED(''), XMLNS('ns1', 'urn:mynamspace:uri'));