nixos/varnish: made compatible with varnish 5.2.1, add modules

* nixos/varnish: command line compatible with varnish 5.2.1, fixes
https://github.com/NixOS/nixpkgs/issues/27409
* nixos/varnish: add support for modules (services.varnish.extraModules)
* varnish-modules: init at 0.10.2
* varnish-geoip: init at 1.0.2
* varnish-rtstatus: init at 1.2.0
* varnish-digest: init at 1.0.1
* added services.varnish.extraCommandLine option