networkd: introduce vti6 tunnel

This patch add support to create vti6 tunnel

test:

vt6.network
[Match]
Name=wlan0

[Network]
Tunnel=ip6vti

vti6.netdev
[NetDev]
Name=ip6vti
Kind=vti6

[Tunnel]
Local=2a00:ffde:4567:edde::4987
Remote=2001:473:fece:cafe::5179

ip link

11: ip6_vti0@NONE: <NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT
group default
    link/tunnel6 :: brd ::
12: ip6vti@wlan0: <POINTOPOINT,NOARP> mtu 1500 qdisc noop state DOWN
mode DEFAULT group default
    link/tunnel6 2a00:ffde:4567:edde::4987 peer 2001:473:fece:cafe::5179