HPCC-10308 Add support for getting ESP files directory listing

URL Format is http://ip:port/esp/files/dirpath?dir&format=json

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Wont catch system vars and things like "~" - Since there could be many variants and change per OS I would suggest gathering the full response as requested and then just check that all the items in the list start with "esp/files"?