Integrate wireguard VPN tunnel with the IPv6 tunnel

[pelzvieh]

In the past, IPsec based products (notably AVM routers) never supported VPN over IPv6 at all, making it pointless to implement IPsec capabilities into IPv6Droid. Things are different with Wireguard, Wireguard tunnels do support IPv6, therefore it should be possible to integrate IPv6Droid tunnels for IPv6 connectivity with Wireguard tunnels for providing VPN on that IPv6 connectivity. From available documentation, this appears feasible, but remains to be proven.