HPCC-11748 Fix WsDFU.DFUQuery not return compressed file size

The existing WsDFU.DFUQuery does not return compressed size. That
is caused by reading a wrong property tree of a logical file. It
is fixed now. The code of WsDFU.DFUFile is changed to return
prefix, nodegroup, and NumParts, which are the same as
WsDFU.DFUQuery.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>
How (from the user of the API) is this different from ShowFileContent?