[thci] add SSH keepalive (#7999)
[thci] add SSH keepalive (#7999)

Add SSH keepalive to avoid disconnection after idle for a long time.
srickardti/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread,jwhui/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread