Add option to declaratively configure visible bridges

[xddxdd]

Add an option to declaratively set the list of bridges visible in Proxmox web interface, so users do not have to manually add the bridge on web interface.

This also helps if /etc/network/interfaces is not persisted across reboots, such as in an impermanence situation.

[JulienMalka]

Sorry for taking a long time to process this PR, I was a bit burried under other things.
I am not sure about this one. I think that's kind of a bandage here and that we should make proxmox able to detect bridges like it is able to detect interfaces, if possible.

[xddxdd]

The problem is that NixOS's networking setup (custom scripts or systemd-networkd) is significantly different from Debian (ifupdown), and making Proxmox load our networking config might require a lot of changes to Proxmox code.

While this PR is just a workaround, it works well (at least for me), and is simple enough so that we can easily remove it if Proxmox provided a proper solution.