Add configuration option to add user to one or more groups.

### Description

It would be nice to have an option in the configuration YAML to add the (non-root) user to one or more groups.

In my specific use-case, I'm trying to create a Debian Bookworm Podman instance and in order for `podman logs` to work properly the non-root user must be added to the `systemd-journal` group.

If you try to do this in the (`system` mode) provisioning script the `usermod` command will run after the user shell has already started and this (typically) requires the VM to be restarted for the change to take effect. If you try in the `boot` mode provisioning script, the user account has not been created yet.

See Discussion #691