bake known checkpoints

I'd like to remove the serialization for this, and turn it into rust code, probably using something like [databake](https://docs.rs/databake/latest/databake/trait.Bake.html), but that's a little heavyweight - we could then just
```rust
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:~
~https://github.com/multiformats/rust-cid/issues/138~
~https://github.com/multiformats/rust-multihash/issues/330~

_Originally posted by @aatifsyed in https://github.com/ChainSafe/forest/pull/3220#discussion_r1267382019_