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/genesis/src/eth1_genesis_service.rs

@@ -114,7 +114,7 @@ impl Eth1GenesisService {
 
         loop {
             let update_result = eth1_service
-                .update_deposit_cache()
+                .update_deposit_cache(None)
                 .await
                 .map_err(|e| format!("{:?}", e));
 
@@ -156,7 +156,7 @@ impl Eth1GenesisService {
             }
 
             // Download new eth1 blocks into the cache.
-            let blocks_imported = match eth1_service.update_block_cache().await {
+            let blocks_imported = match eth1_service.update_block_cache(None).await {
                 Ok(outcome) => {
                     debug!(
                         log,