[enhancement]: Implement ipv6-address-token / ip token set ::123

# Enhancement

In SLAAC deployments, interface tokens are used to override the host-bits with a predictable and short value.
Would be nice to be able to set this token via cloud-init too, to make ipv6-addresses somewhat predictable/readable in dynamic prefix environments.

The resulting configuration would be in netplan:
```yaml
network:
ethernets:
eth0:
ipv6-address-token: ::123
```

/etc/network/interfaces
```
iface eth0 inet6 auto
up ip token set ::123 dev $IFACE
```