Allow installer to use different backends

### Problem

The current installer uses `libdistinst` to install elementary OS packages to disk I believe. Thanks to @jumpyvi we now have a [mkosi/systemd sysupdate](https://github.com/elementary/os/pull/809) PR that can generate an elementary OS 9 image, and then update it forever from there (even over major OS release versions 8=>9=>10!). We need a way to initially copy this image to the users' disk though.

### Proposal

@danirabbit [suggested](https://github.com/elementary/os/pull/809#issuecomment-5115340375) I open a ticket here to start a discussion on the installer supporting different backends.

The installer could work the same as it does now, but on the last step instead of copying packages with `libdistinst` (or `debootstrap`?), do something like:

`sudo dd if=Elementary_build_x86-64.raw of=/dev/sdX bs=4M status=progress conv=fsync`

under the hood. Then reboot into the new image, as it does now.

We could even reuse the current `live-build` live iso generation, but simply copy the elementary OS image to that iso and then have the installer run the above command to the desired disk.

### Prior Art (Optional)

- Gnome OS [proto installer](https://gitlab.gnome.org/GNOME/gnome-build-meta/-/tree/master/files/gnomeos/live/proto-installer?ref_type=heads) - (Rust wrapper around [systemd repart](https://gitlab.gnome.org/GNOME/gnome-build-meta/-/blob/master/files/gnomeos/live/proto-installer/daemon/main.rs#L455) AFAICT)
- KDE Linux ([mkosi + Calamares](https://invent.kde.org/kde-linux/kde-linux/-/tree/master/mkosi.extra/live/usr/calamares?ref_type=heads)) - ([hybrid iso image](https://invent.kde.org/kde-linux/kde-linux/-/blob/master/build.sh#L203) AFAICT)
- https://gitlab.gnome.org/p3732/os-installer
- https://github.com/ublue-os/titanoboa
- https://github.com/FyraLabs/readymade

Start by mapping the current installer path that uses libdistinst, including any debootstrap path, then compare it with the referenced systemd sysupdate/mkosi image flow and live-build generation. Define the backend boundary and image-copy/reboot acceptance criteria before implementing; the issue does not name files or tests.

Written by the indexing model from the issue text.