ot::Ip6::Netif::IterateExternalMulticastAddresses() method

Enables range-based `for` loop iteration over external multicast addresses on the Netif that matches a given IPv6 address type filter. Should be used like follows: to iterate over all external multicast addresses for (Ip6::Netif::ExternalMulticastAddress &addr : Get().IterateExternalMulticastAddresses()) { ... } or to iterate over a subset of external multicast addresses determined by a given address type filter for (Ip6::Netif::ExternalMulticastAddress &addr : Get().IterateExternalMulticastAddresses(Ip6::Address::kTypeMulticastLargerThanRealmLocal)) { ... }