Add IPv6 interface identifier ppp-options
Such options exist for PPPoE-server but not for L2TP
```
vyos@r1# set service pppoe-server ppp-options
Possible completions:
...
ipv6-accept-peer-intf-id
Accept peer interface identifier
ipv6-intf-id Fixed or random interface identifier for IPv6
ipv6-peer-intf-id Peer interface identifier for IPv6
```