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/inner.rs

@@ -1,6 +1,6 @@
 use crate::Config;
 use crate::{
-    block_cache::BlockCache,
+    block_cache::{BlockCache, Eth1Block},
     deposit_cache::{DepositCache, SszDepositCache},
 };
 use parking_lot::RwLock;
@@ -29,6 +29,7 @@ pub struct Inner {
     pub block_cache: RwLock<BlockCache>,
     pub deposit_cache: RwLock<DepositUpdater>,
     pub config: RwLock<Config>,
+    pub remote_head_block: RwLock<Option<Eth1Block>>,
     pub spec: ChainSpec,
 }
 
@@ -86,6 +87,9 @@ impl SszEth1Cache {
                 cache: self.deposit_cache.to_deposit_cache()?,
                 last_processed_block: self.last_processed_block,
             }),
+            // Set the remote head_block zero when creating a new instance. We only care about
+            // present and future eth1 nodes.
+            remote_head_block: RwLock::new(None),
             config: RwLock::new(config),
             spec,
         })