Return eth1-related data via the API (#1797)

## Issue Addressed

- Related to #1691

## Proposed Changes

Adds the following API endpoints:

- `GET lighthouse/eth1/syncing`: status about how synced we are with Eth1.
- `GET lighthouse/eth1/block_cache`: all locally cached eth1 blocks.
- `GET lighthouse/eth1/deposit_cache`: all locally cached eth1 deposits.

Additionally:

- Moves some types from the `beacon_node/eth1` to the `common/eth2` crate, so they can be used in the API without duplication.
- Allow `update_deposit_cache` and `update_block_cache` to take an optional head block number to avoid duplicate requests.

## Additional Info

TBC

beacon_node/eth1/src/deposit_cache.rs

@@ -304,7 +304,7 @@ pub mod tests {
             block_number: 42,
             data: EXAMPLE_LOG.to_vec(),
         };
-        DepositLog::from_log(&log, &spec).expect("should decode log")
+        log.to_deposit_log(&spec).expect("should decode log")
     }
 
     #[test]