add .../apt/update API call

Depends on patched apt-pkg-native-rs. Changelog-URL detection is
inspired by PVE perl code for now, though marked with fixme to use 'apt
changelog' later on, if/when our repos have APT-compatible changelogs
set up.

list_installed_apt_packages iterates all packages and creates an
APTUpdateInfo with detailed information for every package matched by the
given filter Fn.

Sadly, libapt-pkg has some questionable design choices regarding their
use of 'iterators', which means quite a bit of nesting...

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/api2/node.rs

@@ -12,8 +12,10 @@ mod status;
 mod subscription;
 pub(crate) mod rrd;
 pub mod disks;
+mod apt;
 
 pub const SUBDIRS: SubdirMap = &[
+    ("apt", &apt::ROUTER),
     ("disks", &disks::ROUTER),
     ("dns", &dns::ROUTER),
     ("journal", &journal::ROUTER),