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/client/src/builder.rs

@@ -334,6 +334,7 @@ where
                 chain: self.beacon_chain.clone(),
                 network_tx: self.network_send.clone(),
                 network_globals: self.network_globals.clone(),
+                eth1_service: self.eth1_service.clone(),
                 log: log.clone(),
             });
 
@@ -590,7 +591,7 @@ where
                 })?
         };
 
-        self.eth1_service = None;
+        self.eth1_service = Some(backend.core.clone());
 
         // Starts the service that connects to an eth1 node and periodically updates caches.
         backend.start(context.executor);