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

common/eth2/Cargo.toml

@@ -21,7 +21,8 @@ libsecp256k1 = "0.3.5"
 ring = "0.16.12"
 bytes = "0.5.6"
 account_utils = { path = "../../common/account_utils" }
-eth2_ssz = { path = "../../consensus/ssz" }
+eth2_ssz = "0.1.2"
+eth2_ssz_derive = "0.1.0"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 psutil = { version = "3.2.0", optional = true }