net: ipv6: Initial support for privacy extension

This creates support for IPv6 privacy extensions which is described
in RFC 8981. This will also add API that can be used to add IPv6
prefixes to a allow or deny list privacy extension filter.

The code will create temporary IPv6 addresses from prefixes that
are advertised by Router Advertisement messages. The temporary
IPv6 addresses are then expired according to lifetime and removed.

Fixes #9349

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>