OPENTHREAD_CONFIG_CLI_REGISTER_IP6_RECV_CALLBACK macro

@def OPENTHREAD_CONFIG_CLI_REGISTER_IP6_RECV_CALLBACK Define as 1 to have CLI register an IPv6 receive callback using `otIp6SetReceiveCallback()`. This is intended for testing only. Receive callback should be registered for the `otIp6GetBorderRoutingCounters()` to count the messages being passed to the callback.