OPENTHREAD_CONFIG_CHILD_SUPERVISION_INTERVAL macro

@def OPENTHREAD_CONFIG_CHILD_SUPERVISION_INTERVAL The default supervision interval in seconds to use when in child state. Zero indicates no supervision needed. The current supervision interval can be changed using `otChildSupervisionSetInterval()`. Child supervision feature provides a mechanism for parent to ensure that a message is sent to each sleepy child within the supervision interval. If there is no transmission to the child within the supervision interval, child supervisor will enqueue and send a supervision message (a data message with empty payload) to the child.