Major milestone! We're writing ping records to the network, and using
the target's public key to do it!

I think the crypto aspects are more solid now, so I should be able to
move on and focus on getting the ATProto plumbing and interface changes
down.

I'm still not 100% on the crypto here, it seems like maybe I should
start with something simpler for proof-of-concept and if it's really
needed I could always make a v2 record type or something.