ot::DataPollSender::SetExternalPollPeriod() method

Sets/clears a user-specified/external data poll period. Value of zero for `aPeriod` clears the user-specified poll period. If the user provides a non-zero poll period, the user value specifies the maximum period between data request transmissions. Note that OpenThread may send data request transmissions more frequently when expecting a control-message from a parent or in case of data poll transmission failures or timeouts, or when the specified value is larger than the child timeout. A non-zero `aPeriod` should be larger than or equal to `OPENTHREAD_CONFIG_MAC_MINIMUM_POLL_PERIOD` (10ms) or this method returns `kErrorInvalidArgs`. If a non-zero `aPeriod` is larger than maximum value of `0x3FFFFFF ((1 << 26) - 1)`, it would be clipped to this value.