networkd: add GatewayOnLink route option

Adds the GatewayOnLink option in the [Route] section, that allows to bypass the kernel's checking if the gateway may or may not be applicable. The use case is primarily that your IP provider gives you a v4 gateway address that is outside your specific v4 network due to the limited address space, but is reachable anyway.

networkd/templates/20-wired-routed.network.j2

@@ -22,6 +22,13 @@ IPForward=yes
 {% if route.gateway is defined %}
 Gateway={{ route.gateway }}
 {% endif %}
+{% if route.gateway_on_link is defined %}
+{% if ansible_distribution_major_version|int <= 10 %}
+GatewayOnlink={{ route.gateway_on_link }}
+{% else %}
+GatewayOnLink={{ route.gateway_on_link }}
+{% endif %}
+{% endif %}
 {% if route.destination is defined %}
 Destination={{ route.destination }}
 {% endif %}