Feature Request: systemd socket activation

[benubois]

Hello,

Unicorn supported systemd's socket activation feature. This provided a way to achieve zero-downtime restart (besides its native support for the feature). Puma also supports this here.

I was wondering if this could be brought back to pitchfork? It would be great to have another zero-downtime restart option besides going full container/kamal.

Thanks!

[casperisfine]

If it can be implemented without too much changes, I'd merge such pull request.