fix #5254: api: allow usage of download-url with Sys.AccessNetwork

fix #5254: api: allow usage of download-url with Sys.AccessNetwork

The download-url API endpoint has some implications that admins are
unaware of, namely that it basically allow to scan the whole network
via HTTP URLs, and potentially even download some image that the user
should not have access to and adding to a VM that the user controls.

That's why in addition to the Datastore.AllocateTemplate privilege on
the storage, the Sys.Modify on the whole Cluster was required to use
the API call. That design was chosen as we were not fully sure if a
separate privilege is warranted, but user feedback has shown that the
(not so big) cost of adding such a new privilege is justified.

Change the permission check to allow the combination of
Datastore.AllocateTemplate on the storage and either 'Sys.Modify' on
/, for backwards compatibility, or the newer 'Sys.AccessNetwork' on
the node that handles the download.
Using a node-specific ACL path allows admins to e.g. prepare one
specific node's firewall so that pveproxy can access only a safe set
of hosts via outgoing HTTP (not stemming from valid connection
tracking to the PVE API), and thus even further limit the privileges
of users or tools that are trusted to download images to a storage.

Buglink: https://bugzilla.proxmox.com/show_bug.cgi?id=5254
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Tested-by: Hannes Duerr <h.duerr@proxmox.com>
Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>