Introduce `PeerManager::list_peers` and `peer_by_node_id`

Introduce `PeerManager::list_peers` and `peer_by_node_id`

.. returning `PeerDetails` rather than tuples of peer-associated values.

Previously, we wouldn't offer any way to retrieve the features a
peer provided in their `Init` message.

Here, we allow to retrieve them via a new `PeerDetails` struct,
side-by-side with `SocketAddress`es and a bool indicating the direction
of the peer connection.