OPENTHREAD_CONFIG_IPV6_TX_ERR_RATE_AVERAGING_WINDOW macro

@def OPENTHREAD_CONFIG_IPV6_TX_ERR_RATE_AVERAGING_WINDOW OpenThread maintains the average error rate of IPv6 messages per neighbor. This parameter specifies the window (in terms of number of messages) over which the average error rate is maintained. Practically, the average value can be considered as the percentage of failed (no ack) messages over (approximately) last AVERAGING_WINDOW IPv6 messages sent to a specific neighbor.