circuit-v2: Add dialer's multiaddr to StopMessage

[sukunrt]

When making a new connection to a client, the relay server provides the dialer's peer id in the Stop Message.

message StopMessage {
  ...
  optional Peer peer = 2;
  ... other fields
}

If we provide the dialer's address as observed by the relay server, the relay client can gate and limit connections based on the dialer's IP.