feat: update ospf source for learned routes (#388)

- enable changing the source address for learned ospf routes
- this enables the loopback0 interface to be used as a default src address
- ensure k8s nodes use loopback0 as default src
- ensure incus nodes use loopback0 as default src

Reviewed-on: #388

modules/frrouting/templates/frr.conf.erb

@@ -43,3 +43,11 @@ mpls ldp
  exit-address-family
 exit
 <% end -%>
+<% if @ospf_preferred_source_enable && @ospf_preferred_source -%>
+ip prefix-list ANY seq 5 permit 0.0.0.0/0 le 32
+route-map OSPF-SRC permit 10
+ match ip address prefix-list ANY
+ set src <%= @ospf_preferred_source %>
+exit
+ip protocol ospf route-map OSPF-SRC
+<% end -%>