[Request]: Use org.freedesktop.portal.NetworkMonitor on Linux

### Plugin

connectivity_plus

### Use case

I want to distribute my app in a Flatpak sandbox.

The plugin is not allowed to connect to the D-Bus system bus:

```
flutter: SocketException: Connection failed (OS Error: No such file or directory, errno = 2), address = /var/run/dbus/system_bus_socket, port = 0
```

### Proposal

Flatpak portals are a network daemon agnostic way to fetch network information. It works both inside and outside sandboxes.

https://docs.flatpak.org/en/latest/portal-api-reference.html#gdbus-org.freedesktop.portal.NetworkMonitor

It shjould be used instead of trying to talk directly to NetworkManager.