bake known checkpoints

[aatifsyed]

I'd like to remove the serialization for this, and turn it into rust code, probably using something like databake, but that's a little heavyweight - we could then just

struct KnownBlocks<'a> {
    calibnet: &'a [(i64, Cid)],
    mainnet: &'a [(i64, Cid)],
}
const KNOWN_BLOCKS: KnownBlocks<'static> = todo!();

Unfortunately, multihash and cid aren't there yet:
multiformats/rust-cid#138
multiformats/rust-multihash#330

Originally posted by @aatifsyed in #3220 (comment)

[aatifsyed]

Those issues are closed as wrong, we can proceed with this, see multiformats/rust-multihash#331 (comment)