Bluetooth: Mesh: Proxy servers only resends segments to proxy

[trond-snekvik]

The proxy server node first checks whether any connected proxy clients need the packet, and if they do, it won't resend the packet on the advertiser interface. For group dst packets, this is incorrect behavior, as those should go on both interfaces.

Introducing an additional check for unicast address before blocking the adv TX fixes the issue, see #23103.