IPv6oE hosts behind bridged CPEs

This feature adds support for dual-stack IPoE hosts behind bridged clients, receiving globally-routable address using SLAAC or DHCPv6. The feature also provides configurable support for handing out /128 addresses to bridged hosts from same /64 prefix or a unique /64 prefix per host. Bridged hosts that share the same /64 prefix are required to be all SLAAC hosts or DHCPv6 hosts and are required to be associated with the same SAP. For SLAAC based assignment, downstream neighbor-discovery is automatically enabled to resolve the assigned address.