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.

Cargo.toml

@@ -87,6 +87,8 @@ members = [
 
     "validator_client",
     "validator_client/slashing_protection",
+
+    "watch",
 ]
 resolver = "2"