Add PublicAddr option to p2p config

[Voxelot]

Add a --public-addr configuration setting to P2P.

When broadcasting connection info to peers, the public addr setting should be included in the set of "external addresses" to allow for inbound connections from peer discovery via load balancer.

Specifically, the public addr should be loaded into the swarm behavior via this API: https://docs.rs/libp2p-swarm/latest/libp2p_swarm/struct.Swarm.html#method.add_external_address