ot::Utils::Slaac::SetFilter() method

Sets a SLAAC prefix filter handler. The handler is invoked by SLAAC module when it is about to add a SLAAC address based on a prefix. The return boolean value from handler determines whether the address is filtered or added (TRUE to filter the address, FALSE to add address). The filter can be set to `nullptr` to disable filtering (i.e., allow SLAAC addresses for all prefixes).