net: ethernet: Allow drivers to reserve net_pkt headroom

Add new Ethernet driver config option,
ETHERNET_CONFIG_TYPE_EXTRA_TX_PKT_HEADROOM, which allows Ethernet
drivers to inform L2 about the extra net_pkt headroom they need to be
allocated.
This is only supported when CONFIG_NET_L2_ETHERNET_RESERVE_HEADER is
enabled, so that it's possible to fit entire packet into a single
net_buf, which is needed for zero-copy transmission.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>