networkd: add IPv6ProxyNDPAddress support (#5174)

networkd: add IPv6ProxyNDPAddress support (#5174)

IPv6 Neighbor discovery proxy is the IPv6 equivalent to proxy ARP for IPv4.
It is required when ISPs do not unconditional route IPv6 subnets
to their designated target, but expect neighbor solicitation messages
for every address on a link.

A variable IPv6ProxyNDPAddress= is introduced to the [Network] section,
each representing a IPv6 neighbour proxy entry in the neighbour table.