apt: add /changelog API call similar to PVE

apt: add /changelog API call similar to PVE

For proxmox packages it works the same way as PVE, by retrieving the
changelog URL and issuing a HTTP GET to it, forwarding the output to the
client. As this is only supposed to be a workaround removed in the
future, a simple block_on is used to avoid async.

For debian packages we can simply call 'apt-get changelog' and forward
it's output.

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