Incus Executor

Some refactoring and an implementation of the two Incus executors.

This vendors the lxc.generator script from distrobuilder, as data files. Not ideal to have to do this, but I don't see a better option, distrobuilder isn't in Debian.

There are no integration tests, as they probably won't run trivially on salsa-ci. This was why we implemented the unshare backend first.

Incus containers require bootable images, e.g.:

---
bootstrap_options:
  architecture: amd64
  variant: apt
  extra_packages:
    - systemd-sysv
    - systemd
    - dbus
    - systemd-resolved
bootstrap_repositories:
  - mirror: http://deb.debian.org/debian
    suite: bookworm
    components: [main]

Fixes: #271 (closed)