[Discussion] Implement support for other container runtimes

[krishjainx]

Implement container runtime support in the following order:

1. Incus system containers (LXD)
2. Bubblewrap
3. systemd-nspawn

Rationale:

1. Incus now has OCI support, making it a priority
2. A solution using Bubblewrap can be developed
3. nspawn requires dedicated images, which adds complexity to the implementation
4. This can help distrobox provide better security guarantees

Discussed with @89luca89. Self assigning

[Bubbelb]

Note: Systemd-Nspawn also supports OCI Images for a while now: https://www.phoronix.com/news/Systemd-Nspawn-OCI-Runtime

Just for clarity. Also eases installation on a lot of distributions, since systemd is already installed. Though maybe some additional packages are needed (example: in debian systemd-container needs to be installed)