apt: allow changelog retrieval from enterprise repo

apt: allow changelog retrieval from enterprise repo

If a package is or will be installed from the enterprise repo, retrieve
the changelog from there as well (securely via HTTPS and authenticated
with the subcription key).

Extends the get_string method to take additional headers, in this case
used for 'Authorization'. Hyper does not have built-in basic auth
support AFAICT but it's simple enough to just build the header manually.

Take the opportunity and also set the User-Agent sensibly for GET
requests, just like for POST.

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