support systemd socket activation

systemd supports opening a placeholder socket and listen for incoming requests. when a request triggers that socket, systemd passes the file descriptor(no matter its a regular tcp port or ipc file socket) to the application as fd 3, and notifies the application with LISTEN_FDS and LISTEN_PID env variable.

I deploy outline instances on systemd, along with many other services. socket activation helps me keep the service up and running whenever there's a request, without manually starting the service. this also can be used for lazy starting.