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>

src/tools/apt.rs

@@ -128,8 +128,13 @@ fn get_changelog_url(
             None => bail!("incompatible filename, doesn't match regex")
         };
 
-        return Ok(format!("http://download.proxmox.com/{}/{}_{}.changelog",
-                          base, package, version));
+        if component == "pbs-enterprise" {
+            return Ok(format!("https://enterprise.proxmox.com/{}/{}_{}.changelog",
+                              base, package, version));
+        } else {
+            return Ok(format!("http://download.proxmox.com/{}/{}_{}.changelog",
+                              base, package, version));
+        }
     }
 
     bail!("unknown origin ({}) or component ({})", origin, component)