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>

consensus/types/Cargo.toml

@@ -51,6 +51,7 @@ serde_json = "1.0.58"
 criterion = "0.3.3"
 beacon_chain = { path = "../../beacon_node/beacon_chain" }
 eth2_interop_keypairs = { path = "../../common/eth2_interop_keypairs" }
+state_processing = { path = "../state_processing" }
 
 [features]
 default = ["sqlite", "legacy-arith"]