net: shell: Add extra net_pkt TX timing statistics

Add extra net_pkt TX timing information to "net stats" command.
You neeed to enable CONFIG_NET_PKT_TXTIME_STATS and
CONFIG_NET_PKT_TXTIME_STATS_DETAIL config options.
The "net stats" command will print something like this:

Avg TX net_pkt (5867) time 82 us   [0->33->20->27=80 us]

The extra stats information is the end of line inside brackets.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>