Include missing slots instead of skipping the blocks

[fschoell]

Currently whenever a slot is skipped and we get an error when fetching the block number from lighthouse here, we are skipping that block. However skipped blocks are still having states assigned internally, which are needed in certain cases (for example validating blocks). Instead of just skipping those blocks we should include them as missing slots.

Task list:

• [ ] Define a proto for missed slots / integrate it into the current block proto
• [ ] Figure out how to handle block ids and timestamps for missed slots
• [ ] Include that state_root for missed blocks. See if it's queryable from the state endpoints from Lighthouse. If not, recalculate it from the previous blocks

[suchapalaver]

Thanks for opening this! Since I reported the issue, we've open-sourced a tool, a Firehose client in Rust, that we've been using in development for using Firehose in Semiotic's verifiable extraction work. We have a simple workaround when using the Beacon chain addressing this issue, so I thought I'd share that here in case it's useful.