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

Cargo.lock

@@ -363,6 +363,7 @@ dependencies = [
  "derivative",
  "environment",
  "eth1",
+ "eth2",
  "eth2_config",
  "eth2_hashing",
  "eth2_ssz",
@@ -1599,6 +1600,7 @@ version = "0.2.0"
 dependencies = [
  "environment",
  "eth1_test_rig",
+ "eth2",
  "eth2_hashing",
  "eth2_ssz",
  "eth2_ssz_derive",
@@ -1644,6 +1646,7 @@ dependencies = [
  "eth2_keystore",
  "eth2_libp2p",
  "eth2_ssz",
+ "eth2_ssz_derive",
  "hex",
  "libsecp256k1",
  "procinfo",