Implement APTMirror task

https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/aptmirror-task.html specifies a new APT suite mirroring task. Once we have debian:suite collections (#328 (closed)), we should be able to go ahead and implement this task on top of those.

Edited Mar 08, 2024 by Colin Watson

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information