Implement block_rewards API (per-validator reward) (#3907)

## Issue Addressed

[#3661](https://github.com/sigp/lighthouse/issues/3661)

## Proposed Changes

`/eth/v1/beacon/rewards/blocks/{block_id}`

```
{
  "execution_optimistic": false,
  "finalized": false,
  "data": {
    "proposer_index": "123",
    "total": "123",
    "attestations": "123",
    "sync_aggregate": "123",
    "proposer_slashings": "123",
    "attester_slashings": "123"
  }
}
```

The issue contains the implementation of three per-validator reward APIs:
* `sync_committee_rewards`
* [`attestation_rewards`](https://github.com/sigp/lighthouse/pull/3822)
* `block_rewards`

This PR only implements the `block_rewards`.

The endpoints can be viewed in the Ethereum Beacon nodes API browser: [https://ethereum.github.io/beacon-APIs/?urls.primaryName=dev#/Rewards](https://ethereum.github.io/beacon-APIs/?urls.primaryName=dev#/Rewards)

## Additional Info

The implementation of [consensus client reward APIs](https://github.com/eth-protocol-fellows/cohort-three/blob/master/projects/project-ideas.md#consensus-client-reward-apis) is part of the [EPF](https://github.com/eth-protocol-fellows/cohort-three).

Co-authored-by: kevinbogner <kevbogner@gmail.com>
Co-authored-by: navie <naviechan@gmail.com>

common/eth2/src/lib.rs

@@ -1044,6 +1044,22 @@ impl BeaconNodeHttpClient {
         Ok(())
     }
 
+    /// `GET beacon/rewards/blocks`
+    pub async fn get_beacon_rewards_blocks(&self, epoch: Epoch) -> Result<(), Error> {
+        let mut path = self.eth_path(V1)?;
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("beacon")
+            .push("rewards")
+            .push("blocks");
+
+        path.query_pairs_mut()
+            .append_pair("epoch", &epoch.to_string());
+
+        self.get(path).await
+    }
+
     /// `POST beacon/rewards/attestations`
     pub async fn post_beacon_rewards_attestations(
         &self,