Bitcoin block data API

[LLFourn]

We have to decide what functionality we offer the guest machines. The first API we offer them should be useful for stateless machines and we should try and avoid requiring a full spk index for now (so just bitcoin core full node with txindex).

I'd say that we want to figure out some example programs that make sense in first. Perhaps first we should try and use stuff like mempool.space to build them first (since we can already do that with HTTP).