net: icmp: Allow to autogenerate Echo Request payload

Let net_icmpv4_send_echo_request() and net_icmpv6_send_echo_request()
autogenerate Echo Request payload, in case data length is specified but
no data pointer provided. The autogenerated payload includes timestamp
(if payload size permits) so that the turnround time can be calculated.

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