net: icmp: Introduce new API to send ICMP messages

Allow user to send ICMP Echo Request message a.k.a pings.
The same ICMP API and framework is used for network stack
internal needs in later commits. One benefit for this new
API is that it allows sending Echo Requests to offlined
network devices.

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