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);

roles/systemd_networkd/defaults/main.yaml

@@ -0,0 +1 @@
+systemd_networkd__global_config: ""

roles/systemd_networkd/tasks/main.yaml

@@ -12,3 +12,21 @@
     recursive: true
     delete: true
   become: true
+
+- name: ensure global systemd-networkd config directory exists
+  ansible.builtin.file:
+    path: "/etc/systemd/networkd.conf.d"
+    state: directory
+    owner: root
+    group: root
+    mode: "0755"
+  become: true
+
+- name: ensure global systemd-networkd config is deployed
+  ansible.builtin.copy:
+    content: "{{ systemd_networkd__global_config }}"
+    dest: "/etc/systemd/networkd.conf.d/20-ansible.conf"
+    mode: "0644"
+    owner: root
+    group: root
+  become: true