HPCC-12075 Allow to select a good landing zone for Upload/download

When the Upload/download File menu is selected, FileSpray.DropZoneFiles
service tries to retrieve landing zones and the files from the first
landing zone. If the dafilesrv is down on that landing zone, after
several retries for retrieving the files (a long time delay), an
exception is sent out without any landing zone displayed. A user may
not access any landing zone. This fix only retrieves landing zone list
after the Upload/download File menu is clicked. A user may see and
choose any landing zone for file upload and download.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>