Introduce the new `libp2p::SwarmBuilder`. Users should use the new `libp2p::SwarmBuilder` instead of the now deprecated `libp2p::swarm::SwarmBuilder`. See `libp2p::SwarmBuilder` docs on how to use the new builder.
Fixes#3657.
Fixes#3563.
Fixes#3179.
Pull-Request: #4120.
Implements UPnP via the IGD protocol. The usage of IGD is an implementation detail and is planned to be extended to support NATpnp.
Resolves: #3903.
Pull-Request: #4156.