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>