nixos/send: Add `environmentFile` option for securely passing secrets (#462978)

        };

      };

      environmentFile = mkOption {

        default = null;

        description = ''

          Environment file (see {manpage}`systemd.exec(5)` "EnvironmentFile="

          section for the syntax) passed to the service. This option is the

          recommended way to pass secrets to Send.

          This is especially important for users using a cloud storage backend.

          A list of environment variables recognized by Send can be found here:

          <https://github.com/timvisee/send/blob/master/docs/docker.md>

        '';

        example = "/run/secrets/send";

        type = with types; nullOr path;

      };

      dataDir = lib.mkOption {

        type = types.path;

        readOnly = true;

        LoadCredential = lib.optionalString (

          cfg.redis.passwordFile != null

        ) "redis-password:${cfg.redis.passwordFile}";

        EnvironmentFile = cfg.environmentFile;

        # Hardening

        RestrictAddressFamilies = [