systemd_networkd(role),router(host): support global config to fix forw.

With the router upgrade to Debian 13 the systemd version got upgraded as
well breaking the current configuration for IP forwarding.
Add a variable for global systemd-networkd configuration and use that to
enable IPv4 and IPv6 forwarding on the router.

The systemd_networkd role could be a bit nicer, not deploying/deleting
the global configuration, if the variable is empty and
reloading/restarting systemd-networkd at appropriate times. But as is
works for now.

roles/systemd_networkd/README.md

@@ -9,3 +9,8 @@ Should work on Debian-based distributions.
 ## Required Arguments
 
 - `systemd_networkd__config_dir`: Directory with systemd-networkd configs to deploy.
+
+## Optional Arguments
+
+- `systemd_networkd__global_config`: systemd-networkd global configuration to deploy (see `man 5 networkd.conf`).  
+  Defaults to `` (the empty string);