Intergration into libp2p

[dariusc93]

This implementation relies 2 depenedencies igd-next and natpmp, with the latter being used as a fallback if igd fails. This behaviour works almost out of the box.

Currently, this solution may not be suitable to be apart of the libp2p transport itself as it relies on separate task with channels to communicate, as well as natpmp relies on FFI calls, which I believe rust-libp2p may not want to be apart of their codebase.

Thoughts for the future (may change over time):

• Implement NATPMP (as well as PCP) in rust, with attempts not to rely on FFI calls to C code.
• Simplify IGD as much as possible to reduce its dependency footprint (although it should be fine as is, but less dependencies is usually the better).

Reference:

• NAT traversal libp2p/rust-libp2p#2052
• UPnP in the TcpConfig libp2p/rust-libp2p#558