Add new Snapshot= setting

Add new Snapshot= setting

In systemd CI, we often run into issues caused by updates to third-party
components like the kernel package in rolling release distributions
like Arch Linux or Fedora Rawhide. When these happen, the corresponding
CI job starts failing on every PR and bisecting the distribution to figure
out when the breakage was introduced is rather tedious.

To mitigate this problem, we need to be able to pin the rolling release
distributions to a specific snapshot which we control. This allows us to
update the pinned snapshot in a PR created by a bot, so that any failures
introduced by moving to a newer snapshot will be limited to the PR that bumps
the snapshot. Any regressions can then be debugged and fixed before merging
the PR that switches us to the new snapshot.

To make this possible, let's introduce a new Snapshot= setting and implement
it for every distribution that has a snapshot concept or something that maps
to it. Per distribution:

- Debian => snapshot.debian.org (unlimited)
- Ubuntu => snapshot.ubuntu.com (unlimited)
- Arch => archive.archlinux.org (unlimited)
- OpenSUSE => download.opensuse.org/history (limited to a month of snapshots)
- CentOS => composes.stream.centos.org (limited to 3 weeks of snapshots)
- Fedora => https://kojipkgs.fedoraproject.org (limited to 2 weeks of snapshots)

Additionally, for CentOS, we also support using composes from mirror.facebook.net
which keeps them around forever so we get unlimited snapshots there as well for
CentOS Stream.

We also add a latest-snapshot verb to be able to easily figure out the latest
snapshot so it can be bumped regularly via a CI workflow. Because we do not track
sufficient information from config files to be able to insert the updated snapshot
into the right config file ourselves, we output it on stdout instead and leave it to
users to insert it into the right config file.