Add support for reading interface and sub-interface Ipv4/Ipv6 neighbours.

When interface configuration contains:
{
"name": "GigabitEthernet0/8/0",
"ietf-ip:ipv4": {
"neighbor": [

{"ip": "192.168.0.4", "link-layer-address": "08:00:27:c0:5d:37"}

],
"address": [

{"ip": "192.168.0.2", "netmask": "255.255.255.0"}

]
}
}

Then reading interface operational data results in:
{
"name":"GigabitEthernet0/8/0",
"ietf-ip:ipv4":{
"address":[

{"ip":"192.168.0.2","prefix-length":24}

]
}

IP neighbor is missing from operational data. Also applies to ipv6 neighbours, and also applies to ipv4/ipv6 in sub-interfaces.