Add a DHT whose entries are keys, and whose associated values are a list of PeerId and listening addresses of nodes which own the key. The entries are signed.

This should be done by modifying network-libp2p. We can add a method to Service to start a request for a specific key. On success, we should automatically connect to the node which owns the key. The OpenCustomProtocol that reports the successful connection should indicate that this corresponds to a fulfilled request. Also add a new variant in ServiceEvent for requests that time out or fail.