[Request]: connectivity_plus on linux: use systemd-networkd as fallback when network-manager isn't available

### Plugin

connectivity_plus

### Use case

connectivity_plus currently relies solely on network-manager to check for a connection under linux. If network-manager isn't available or the network is managed in a different way, connectivity_plus will always think there's no connection available.
See https://github.com/fluttercommunity/plus_plugins/issues/1304, https://github.com/ubuntu-flutter-community/software/issues/674

### Proposal

It would be great to support some alternatives that can be used as a fallback in case network-manager isn't available. A sensible option would be systemd-networkd, which can be accessed through the [`org.freedesktop.network1` DBus interface](https://www.freedesktop.org/software/systemd/man/org.freedesktop.network1.html).