status: add download_url method

status: add download_url method

uses common function PVE::Tools::download_file_from_url to download
iso files.

Only users with permissions `Sys.Audit` and `Sys.Modify` on `/` are
permitted to perform this action. This restriction is due to the
fact, that the download function is able to download files from
internal networks (which are not visible/accessible from outside).
Users with these permissions anyway have the means to alter node
(network) config, so this does not create any further security risk.

Signed-off-by: Lorenz Stechauner <l.stechauner@proxmox.com>