Pkarr based node discovery for iroh-net

Node discovery is being able to find connecting information about an iroh node based on just its node id.

This crate implements a discovery mechanism for iroh-net based on https://pkarr.org/.

TLDR: Each node publishes its address to the mainline DHT as a DNS packet, signed with its private key. The DNS packet contains the node's direct addresses and optionally a DERP URL.