Code review comment for ~rmalz/charm-advanced-routing:rmalz/modify_networkd_conf

Hey Ed,
Following proposal I have created new file based on [1].
Script will create a new file in /usr/lib/systemd/networkd.conf.d/juju-networkd.conf" and always overwrite it with:
root@juju-946413-myostmodel-21:/home/ubuntu# cat /usr/lib/systemd/networkd.conf.d/juju-networkd.conf
# This file is managed by Juju.
[Network]
ManageForeignRoutingPolicyRules=no
ManageForeignRoutes=no

I have tested this locally, works fine.

[1] https://manpages.ubuntu.com/manpages/jammy/man5/networkd.conf.5.html