Add dropin file support for `.network` files for networkd

[choffee]

Affected Puppet, Ruby, OS and module versions/distributions

• Puppet: 7
• Ruby: 2.7
• Distribution: Ubuntu
• Module version: 7.1.0

Since systemd 232 it has supported dropin files for network interfaces. It would be good to get support here for those as well.

eg /etc/systemd/network/eth0.network.d/routes.conf

[kenyon]

I guess the implication is that you're not puppet-managing the .network files, but you just want to puppet-manage some modifications using the dropin. I haven't found a need for this because I have been managing the .network files, but I suppose this could be useful in some use cases. It's analogous to managing dropin files for service units, but not managing service units themselves because the package management system does that.