net/icmpv6: Allow for arbitrary payload data in ICMP echo

Allow for including arbitrary data in net_icmpv6_send_echo_request()
that will be echoed verbatim by the receiver.

This allows to use ICMP echo for diagnostic use cases, e.g. by testing
packet framentation (large payload) or measuring round-trip-time.

Signed-off-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>