Implement checkpoint sync (#2244)

## Issue Addressed

Closes #1891
Closes #1784

## Proposed Changes

Implement checkpoint sync for Lighthouse, enabling it to start from a weak subjectivity checkpoint.

## Additional Info

- [x] Return unavailable status for out-of-range blocks requested by peers (#2561)
- [x] Implement sync daemon for fetching historical blocks (#2561)
- [x] Verify chain hashes (either in `historical_blocks.rs` or the calling module)
- [x] Consistency check for initial block + state
- [x] Fetch the initial state and block from a beacon node HTTP endpoint
- [x] Don't crash fetching beacon states by slot from the API
- [x] Background service for state reconstruction, triggered by CLI flag or API call.

Considered out of scope for this PR:

- Drop the requirement to provide the `--checkpoint-block` (this would require some pretty heavy refactoring of block verification)


Co-authored-by: Diva M <divma@protonmail.com>

book/src/SUMMARY.md

@@ -30,6 +30,7 @@
 		* [Signature Header](./api-vc-sig-header.md)
 	* [Prometheus Metrics](./advanced_metrics.md)
 * [Advanced Usage](./advanced.md)
+    * [Checkpoint Sync](./checkpoint-sync.md)
     * [Custom Data Directories](./advanced-datadir.md)
     * [Validator Graffiti](./graffiti.md)
     * [Remote Signing with Web3Signer](./validator-web3signer.md)