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/state_processing/src/per_block_processing.rs

@@ -11,7 +11,7 @@ pub use self::verify_attester_slashing::{
 };
 pub use self::verify_proposer_slashing::verify_proposer_slashing;
 pub use altair::sync_committee::process_sync_aggregate;
-pub use block_signature_verifier::BlockSignatureVerifier;
+pub use block_signature_verifier::{BlockSignatureVerifier, ParallelSignatureSets};
 pub use is_valid_indexed_attestation::is_valid_indexed_attestation;
 pub use process_operations::process_operations;
 pub use verify_attestation::{