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>

beacon_node/client/src/config.rs

@@ -1,5 +1,6 @@
 use directory::DEFAULT_ROOT_DIR;
 use network::NetworkConfig;
+use sensitive_url::SensitiveUrl;
 use serde_derive::{Deserialize, Serialize};
 use std::fs;
 use std::path::PathBuf;
@@ -9,7 +10,7 @@ use types::{Graffiti, PublicKeyBytes};
 const DEFAULT_FREEZER_DB_DIR: &str = "freezer_db";
 
 /// Defines how the client should initialize the `BeaconChain` and other components.
-#[derive(PartialEq, Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum ClientGenesis {
     /// Creates a genesis state as per the 2019 Canada interop specifications.
     Interop {
@@ -26,6 +27,15 @@ pub enum ClientGenesis {
     /// We include the bytes instead of the `BeaconState<E>` because the `EthSpec` type
     /// parameter would be very annoying.
     SszBytes { genesis_state_bytes: Vec<u8> },
+    WeakSubjSszBytes {
+        genesis_state_bytes: Vec<u8>,
+        anchor_state_bytes: Vec<u8>,
+        anchor_block_bytes: Vec<u8>,
+    },
+    CheckpointSyncUrl {
+        genesis_state_bytes: Vec<u8>,
+        url: SensitiveUrl,
+    },
 }
 
 impl Default for ClientGenesis {