Support loading password from file

[zoechi]

• I try to use sops-nix to pass the secret (password) to go2rtc.
• I can't assign an owner to the created secret file, because the dynamic systemd user is not available at deployment time.
• If I pass the secret using systemd's LoadSecret the go2rtc service becomes the owner of the secret file, but go2rtc needs it as environment variable and there is no sane way to do that.

This becomes extremly cumbersome

See also

• RFE: credentials loaded with LoadCredential should be available to ExecStartPre too systemd/systemd#19604 (comment)
• Access secret from systemd service with DynamicUser=true Mic92/sops-nix#198