Feature request: support IPIP tunnels

setting IPIP tunnels with nmcli is possible as eg:
```
nmcli connection add type ip-tunnel ip-tunnel.mode ipip con-name tun0 ifname tun0 local x.x.x.x remote 0.0.0.0
nmcli connection modify tun0 ipv4.addresses 'y.y.y.y/zz'
nmcli connection modify tun0 ipv4.method manual
nmcli connection up tun0
```
We would like to be able to do this through nmstate too
