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/block_cache.rs

@@ -1,6 +1,7 @@
 use ssz_derive::{Decode, Encode};
 use std::ops::RangeInclusive;
-use types::{Eth1Data, Hash256};
+
+pub use eth2::lighthouse::Eth1Block;
 
 #[derive(Debug, PartialEq, Clone)]
 pub enum Error {
@@ -15,28 +16,6 @@ pub enum Error {
     Internal(String),
 }
 
-/// A block of the eth1 chain.
-///
-/// Contains all information required to add a `BlockCache` entry.
-#[derive(Debug, PartialEq, Clone, Eq, Hash, Encode, Decode)]
-pub struct Eth1Block {
-    pub hash: Hash256,
-    pub timestamp: u64,
-    pub number: u64,
-    pub deposit_root: Option<Hash256>,
-    pub deposit_count: Option<u64>,
-}
-
-impl Eth1Block {
-    pub fn eth1_data(self) -> Option<Eth1Data> {
-        Some(Eth1Data {
-            deposit_root: self.deposit_root?,
-            deposit_count: self.deposit_count?,
-            block_hash: self.hash,
-        })
-    }
-}
-
 /// Stores block and deposit contract information and provides queries based upon the block
 /// timestamp.
 #[derive(Debug, PartialEq, Clone, Default, Encode, Decode)]
@@ -55,6 +34,16 @@ impl BlockCache {
         self.blocks.is_empty()
     }
 
+    /// Returns the earliest (lowest timestamp) block, if any.
+    pub fn earliest_block(&self) -> Option<&Eth1Block> {
+        self.blocks.first()
+    }
+
+    /// Returns the latest (highest timestamp) block, if any.
+    pub fn latest_block(&self) -> Option<&Eth1Block> {
+        self.blocks.last()
+    }
+
     /// Returns the timestamp of the earliest block in the cache (if any).
     pub fn earliest_block_timestamp(&self) -> Option<u64> {
         self.blocks.first().map(|block| block.timestamp)
@@ -181,6 +170,7 @@ impl BlockCache {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use types::Hash256;
 
     fn get_block(i: u64, interval_secs: u64) -> Eth1Block {
         Eth1Block {