Add Option<Vec<SocketAddress>> to OnionMessagePath

Add Option<Vec<SocketAddress>> to OnionMessagePath

MessageRouter::find_path is given a Destination to reach via a set of
peers. If a path cannot be found, it may return a partial path such that
OnionMessenger can signal a direct connection to the first node in the
path is needed. Include a list of socket addresses in the returned
OnionMessagePath to allow OnionMessenger to know how to connect to the
node.

This allows DefaultMessageRouter to use its NetworkGraph to return
socket addresses for gossiped nodes.