HPCC-8845 Support XMLNS for workunit and roxie xml outputs

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'));

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
HPCC-8845 Changes based on code review

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
HPCC-8845 Update based on review, and add unicode test case

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #5800 from afishbeck/outputNamespaces

HPCC-8845 Support XMLNS for workunit and roxie xml outputs

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>