Massive first commit. Takes the atproto OAuth code from annot.at, but wires it up as a library. I think I've already decided to change directions here. This is a stateless library, but I think it's hard to make a really nice stateless atproto OAuth library. Adding some state means we can do DPoP nonce caching and stuff. This also introduces proper Error structs on the annot.at code to make it nicer to use as a library.