net: shell: Add net-sockets command

The new "net sockets" command will utilize the object core
support to track and show information about BSD sockets that
are created in the system. This command is able to show info
for all network sockets (native, offloaded etc) in the system.

Example of the output of the new command:

uart:~$ net sockets
 Creator  Name       Flags  FD   Lifetime (ms) Sent  Received

    main  af_inet46  6ST    0    3260          819   498
    main  af_inet46  4ST    1    2110          469   142
    main  af_inet46  6DU    2    2110          9941  9941
    main  af_inet46  4DU    3    2110          1375  621

4 active sockets found.

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