[router-table] simplify `Allocate()` and random selection of ID (#8032)
[router-table] simplify `Allocate()` and random selection of ID (#8032)

This commit updates `RouterTable::Allocate()` to use Reservoir
sampling algorithm to randomly select a router ID while iterating
through the list of available IDs.
openthread/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread,jwhui/openthread,jwhui/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread