Provide a shell variable for network meter status #673

Is your feature request related to a problem? Please describe.
There is no shell variable to query network metered status. For example, I want to create a backup locally, while also uploading the backup to a generic Internet server. There is no way for me to find out whether the network is metered, without doing some long OS-specific scripting, while Vorta already knows the metered status of the network.

Describe the solution you'd like
Add a shell variable. something like $metered to query the status. We already have an is_network_metered method, so it should just call that.

Issue is communicating the null state, when Vorta doesn't know if the network is metered or not.

Describe alternatives you've considered
Use OS-specific methods to get the metered status.