net: tracing: Add TX time tracing

TX time tracing tells how long it took from network packet
creation to when the stack got rid of it.
So the network stack allocates net packet, this is the
start time. The end time is when the packet is fully processed (sent)
by the network device driver.

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