Proxy to use to connect to the wstunnel server (USER:PASS@HOST:PORT).
Passwords specified here will be world-readable in the Nix store! To pass a password to the service, point the
environmentFile option to a file containing PROXY_PASSWORD=<your-password-here> and set this option to <user>:$PROXY_PASSWORD@<host>:<port>. Note however that this will also locally leak the passwords at runtime via e.g. /proc/<pid>/cmdline.