OPENTHREAD_CONFIG_ANNOUNCE_SENDER_INTERVAL macro

@def OPENTHREAD_CONFIG_ANNOUNCE_SENDER_INTERVAL Specifies the time interval (in milliseconds) between `AnnounceSender` transmit cycles. In a cycle, the `AnnounceSender` sends MLE Announcement on all channels in Active Operational Dataset's ChannelMask. The transmissions on different channels happen uniformly over the given interval (i.e., if there are 16 channels, there will be 16 MLE Announcement messages each on one channel with `interval / 16` between two consecutive MLE Announcement transmissions). Applicable only if `AnnounceSender` feature is enabled (see `OPENTHREAD_CONFIG_ANNOUNCE_SENDER_ENABLE`).