Add multi-transport support to ``Swarm``, ``new_swarm``, and ``new_host``.
A node can now listen and dial over TCP, WebSocket, and QUIC simultaneously,
matching go-libp2p's ``TransportManager`` architecture.  Pass an explicit
``transports=[...]`` list to ``new_swarm`` / ``new_host``, or let transports
be auto-detected from ``listen_addrs``.