Support client authentication

[michaelweiser]

This issue is to track our need and implementation of some kind of client authentication, as was recently highlighted again in scVENUS/PeekabooAV-Installer#28. This is independen from protocol changes (i.e. #77) since it also applies to our current UNIX domain socket. A first attempt could be as simple as a single password, i.e. API token, for all clients.