Implement API for block rewards (#2628)

## Proposed Changes

Add an API endpoint for retrieving detailed information about block rewards.

For information on usage see [the docs](https://github.com/sigp/lighthouse/blob/block-rewards-api/book/src/api-lighthouse.md#lighthouseblock_rewards), and the source.

beacon_node/beacon_chain/src/lib.rs

@@ -5,6 +5,7 @@ mod beacon_chain;
 mod beacon_fork_choice_store;
 mod beacon_proposer_cache;
 mod beacon_snapshot;
+pub mod block_reward;
 mod block_times_cache;
 mod block_verification;
 pub mod builder;