Expose the PCP from the networking code

[MOZGIII]

Hey guys! I was looking around and found your Port Control Protocol implementation. See, I'd really like to use it, and I think I can via the iroh-net crate, but I really only want the port mapper functionality.

Would you consider extracting your PCP implementation as a separate crate? I think, for the most flexibility, this part of the dependency tree should look somewhat like this: iroh-net -> portmapper -> pcp - where each of these is a separate crate, and the arrow is a dependency relation, where the crate on the left of the arrow depends on the crate on the right of the arrow.

[divagant-martian]

https://crates.io/crates/pcp-client