Add beacon.watch (#3362)

> This is currently a WIP and all features are subject to alteration or removal at any time.

## Overview

The successor to #2873.

Contains the backbone of `beacon.watch` including syncing code, the initial API, and several core database tables.

See `watch/README.md` for more information, requirements and usage.

beacon_node/http_api/src/lib.rs

@@ -18,6 +18,7 @@ mod standard_block_rewards;
 mod state_id;
 mod sync_committee_rewards;
 mod sync_committees;
+pub mod test_utils;
 mod ui;
 mod validator_inclusion;
 mod version;