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_log.rs

@@ -1,11 +1,12 @@
 use super::http::Log;
 use ssz::Decode;
-use ssz_derive::{Decode, Encode};
 use state_processing::per_block_processing::signature_sets::{
     deposit_pubkey_signature_message, deposit_signature_set,
 };
 use types::{ChainSpec, DepositData, Hash256, PublicKeyBytes, SignatureBytes};
 
+pub use eth2::lighthouse::DepositLog;
+
 /// The following constants define the layout of bytes in the deposit contract `DepositEvent`. The
 /// event bytes are formatted according to the  Ethereum ABI.
 const PUBKEY_START: usize = 192;
@@ -19,22 +20,10 @@ const SIG_LEN: usize = 96;
 const INDEX_START: usize = SIG_START + 96 + 32;
 const INDEX_LEN: usize = 8;
 
-/// A fully parsed eth1 deposit contract log.
-#[derive(Debug, PartialEq, Clone, Encode, Decode)]
-pub struct DepositLog {
-    pub deposit_data: DepositData,
-    /// The block number of the log that included this `DepositData`.
-    pub block_number: u64,
-    /// The index included with the deposit log.
-    pub index: u64,
-    /// True if the signature is valid.
-    pub signature_is_valid: bool,
-}
-
-impl DepositLog {
+impl Log {
     /// Attempts to parse a raw `Log` from the deposit contract into a `DepositLog`.
-    pub fn from_log(log: &Log, spec: &ChainSpec) -> Result<Self, String> {
-        let bytes = &log.data;
+    pub fn to_deposit_log(&self, spec: &ChainSpec) -> Result<DepositLog, String> {
+        let bytes = &self.data;
 
         let pubkey = bytes
             .get(PUBKEY_START..PUBKEY_START + PUBKEY_LEN)
@@ -68,7 +57,7 @@ impl DepositLog {
 
         Ok(DepositLog {
             deposit_data,
-            block_number: log.block_number,
+            block_number: self.block_number,
             index: u64::from_ssz_bytes(index).map_err(|e| format!("Invalid index ssz: {:?}", e))?,
             signature_is_valid,
         })
@@ -77,7 +66,6 @@ impl DepositLog {
 
 #[cfg(test)]
 pub mod tests {
-    use super::*;
     use crate::http::Log;
     use types::{EthSpec, MainnetEthSpec};
 
@@ -113,6 +101,7 @@ pub mod tests {
             block_number: 42,
             data: EXAMPLE_LOG.to_vec(),
         };
-        DepositLog::from_log(&log, &MainnetEthSpec::default_spec()).expect("should decode log");
+        log.to_deposit_log(&MainnetEthSpec::default_spec())
+            .expect("should decode log");
     }
 }