Expose information about pending and failed connections

[Frando]

Currently, iroh-gossip neither exposes how many join connections are currently pending (dialing), nor does it provide an event in case a connection fails. This makes managing neighbors e.g. by adding more peers from a list of bootstrap peers unnecessarily difficult.

iroh-gossip should:

• Emit an event if the connection to a peer added via join_peers or as part of the bootstrap peers failed
• Expose the number of currently-pending connections