Feature Request: Add support for systemd socket activation

[claudio4]

Hello, I would like to propose adding support for SystemD socket activation.

This brings the following advantages:

• Enables delayed program startup, allowing Quantum to run only when a connection is requested.
• No 502 errors during program restarts, as Systemd keeps the socket open, resulting in just a slightly longer load time for clients while the service restarts.
• Free Unix socket support, meaning connections can be managed with classic Unix file permissions, eliminating the "which port did I use" guessing game and offering a minor performance boosts when using a reverse proxy on the same machine.
• Improved performance with Podman, as socket activation bypasses the containerized network, making it as efficient as using the host network.
• With Podman, containers can run without a network since socket activation doesn't depend on it.
• It makes long-bearded Linux admins happy.

I imagine this addition would introduce minimal code complexity since Go's net package handles much of the intricacies of various socket types. Additionally, libraries like CoreOS's go-systemd could simplify the process even more.

If there's interest in this feature, I'd be happy to volunteer to write a PR.

[gtsteffaniak]

This all sounds great, I don't see any reason it can't be added as long as it does impact users that don't need the feature.

I'm not too familiar with this, so any PR would be much appreciated, even if it's not fully working it would help me understand the intention