Autonat support

[achingbrain]

An implementation of Autonat compatible with https://github.com/libp2p/specs/tree/master/autonat and https://github.com/libp2p/go-libp2p-autonat is required.

See also: https://docs.libp2p.io/concepts/nat/

[TinyTb]

backlog grooming: nearly done, but needs some time spent on it.