Use STUN server to speed up the external socket address discovery

[carver]

Add a client like stunclient to trin, and run rustun server with something like:

cargo run --release --example binding_srv

On launch, we can check our external address to create an ideal ENR much faster than waiting for a bunch of independent PONGs to come in. (It also simplifies bootstrapping, where we launch new peers and connect them to each other)