net: pkt_filter: Introduce additional hooks for pkt_filter

The additional hooks provide infrastructure to construct
rules on another network stack levels. Main benefit of this
approach is packets are pre-parsed and e.g. IP filter is
easier to implement. These hooks are equivalent of prerouting
and local_in in linux's netfilter.

Signed-off-by: Marcin Gasiorek <marcin.gasiorek@nordicsemi.no>