net: Extend the protocol handling in Ethernet

Allow user to specify protocol extensions when receiving data
from Ethernet network. This means that user can register L3
protocol handler using NET_L3_REGISTER() with the desired
protocol type. Ethernet code will then call the handler if
such a protocol type packet is received. This is currently
only implemented for Ethernet. The original IPv4 and IPv6
handling is left intact even if they can be considered to
be L3 layer protocol. This could be changed in the future
if needed so that IPv4 and IPv6 handling could be made
pluggable protocols.

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