Add possibility to use systemdspawner as non-root

Hi,
is there any chance to add sth. like the [nonrootv2 feature](https://gitlabph.physik.fu-berlin.de/behrmann/systemdspawner/-/tree/nonrootv2) to systemdspawner?

Being able to have juypterhub run as non-root is the only reason for the sudo spawner, which misses all the config options that systemdspawner has. The mechanism in Joergs nonrootv2 tree adds a very systemd'ish way to use per-user service file instances.

Have you considered adding this? Would be great to have systemdspawner + non-root...
cu,
Frank