Add option to configure autoconf (slaac) for ipv6 interfaces

[strifel]

SUMMARY

Allow to specify autoconf as ip address for v6 interfaces.

ISSUE TYPE

• Feature Idea

COMPONENT NAME

vyos_l3_interfaces

ADDITIONAL INFORMATION

https://docs.vyos.io/en/equuleus/configuration/interfaces/ethernet.html#cfgcmd-set-interfaces-ethernet-interface-ipv6-address-autoconf

[abaschen]

Similarly dhcpv6-options