IPV6: test client can work with IPV6, in addition to IPV4

[glimchb]

• add client support if needed
• add ci (dhcp server) support
• add tests

Use DHCP option 143 (IPv4) or 163 (IPv6), as described in RFC 8572

need changes here: https://github.com/opiproject/sztp/blob/302b5b740cbc5562596cf9eea389cc9b876f3da2/dhcp/dhcpd.conf.template#L24