Add IPv6 support for LAN to show IPv6 leases and so on
User on our forum [requested](https://forum.turris.cz/t/turris-os-5-0-0-is-going-to-be-in-hbk/12445/9?u=pepe) to have more IPv6 stuff to configure and show in UI.
I take a look at LAN backend:
https://gitlab.labs.nic.cz/turris/foris-controller/foris-controller/blob/master/foris_controller_backends/lan/__init__.py#L42
and I was able to see that there is just IPv4 support.
IPv6 leases are stored in file `/var/hosts/odhcdp`. It will allows us to show and configure IPv6 network in LAN. For example to have something like LuCI has:

Currently, we provide to way a configure just IPv4 network and show leases from it
issue