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>

lcli/Cargo.toml

@@ -15,7 +15,8 @@ hex = "0.4.2"
 log = "0.4.11"
 serde = "1.0.116"
 serde_yaml = "0.8.13"
-simple_logger = "1.10.0"
+serde_json = "1.0.66"
+env_logger = "0.9.0"
 types = { path = "../consensus/types" }
 state_processing = { path = "../consensus/state_processing" }
 eth2_ssz = "0.3.0"