HPCC-11674 Return WU XML and ECL in WsWorkunits/WUFile

Two groups of functions are added into WUFile by this fix: 1. a user
may download WU XML into a text file; 2. a user may retrieve WU ECL
or download it into a text file or a zip file (Type = 'WUECL').

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>