Add support for socket redirection to stdin/stdout

[ueno]

The goal of this issue is make it possible to write a socket-activatable service as in systemd's ListenStream=. To allow that:

• the config file needs to be extended to have a new enum value in Stdio, say Listen, to indicate the use of this feature
• enarx-wasmldr needs to have a capability of listening on a socket and activate (run) the workload when a new connection comes