Add support for phone side telephony plugin on Linux.

## Summary

Add support for Phone side (like in the Android and SailfishOS apps) to telephony plugin.
this requires ModemManager to work.
Call state notifications are currently supported, more capabilities are possible.

Tested on PinePhone but should probably work on every linux Phone with ModemManager.
to test, pair kdeconnect on your linux phone to desktop, and call your phone.
you should get notification on the desktop, telling you that phone call is incoming.