Implement Nat traversal

[dryajov]

[sinkingsugar]

Would adding upnp/nat-pmp support be enough or actually we are talking of stuff like STUN ( would require UDP transport too for example) ?

[arnetheduck]

upnp/natpmp would be enough - we're using https://github.com/status-im/nim-nat-traversal - at some point in a distant future we might do a pure nim version as well, but that's not too urgent.

I'd actually really want to see a udp-based transport as well - and there are efforts in the libp2p community to define what the interfaces to such transports should look like so they remain interoperable - we'd do well in joining those efforts

[Menduist]

Hole-Punching has been implemented for TCP: #826

Remains to add a UPnP / PmP solution, like nim-nat-traversal, or nim-nat-mapper (a WiP pure nim library)