T4832: dhcp: Add IPv6-only dhcp option support (RFC 8925)

Clients supporting this DHCP option (DHCP option 108, RFC 8925) will
disable its IPv4 network stack for configured number of seconds
and operate in IPv6-only mode.

This option is known to work on iOS 15+ and macOS 12.0.1+.

Example command:

set service dhcp-server shared-network-name LAN6 subnet 192.168.64.0/24 ipv6-only-preferred 0