net: pkt: Add IP DSCP/ECN information to a packet structure

Add new fields to the net_pkt structure, representing IP-specific
Differentiated services code point (DSCP) and Explicit Congestion
Notification (ECN) values. Those values are encoded in legacy
Type of Service (IPv4) and Trafic Class (IPv6) header fields.

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