Make litep2p aware of external addresses

[dmitry-markin]

Currently, litep2p knows only about listen addresses of the node. External addresses handling is left to the client code.

Make litep2p aware of external addresses by introducing API for the client code to notify litep2p about confirmed public addresses.

This is needed to implement #191 and to report to the network the addresses of local node acting as a content provider in Kademlia #196.

Possible next steps

• Move the heuristic of confirming external addresses seen by three or more peers from polkadot-sdk into litep2p.