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/errors.rs

@@ -137,6 +137,9 @@ pub enum BeaconChainError {
     AltairForkDisabled,
     ExecutionLayerMissing,
     ExecutionForkChoiceUpdateFailed(execution_layer::Error),
+    BlockRewardSlotError,
+    BlockRewardAttestationError,
+    BlockRewardSyncError,
     HeadMissingFromForkChoice(Hash256),
     FinalizedBlockMissingFromForkChoice(Hash256),
     InvalidFinalizedPayloadShutdownError(TrySendError<ShutdownReason>),