Implement attestation_rewards API (per-validator reward) (#3822)

## Issue Addressed

#3661 

## Proposed Changes
`/eth/v1/beacon/rewards/attestations/{epoch}`

```json
{
  "execution_optimistic": false,
  "finalized": false,
  "data": [
    {
      "ideal_rewards": [
        {
          "effective_balance": "1000000000",
          "head": "2500",
          "target": "5000",
          "source": "5000"
        }
      ],
      "total_rewards": [
        {
          "validator_index": "0",
          "head": "2000",
          "target": "2000",
          "source": "4000",
          "inclusion_delay": "2000"
        }
      ]
    }
  ]
}
```

The issue contains the implementation of three per-validator reward APIs:
- [`sync_committee_rewards`](https://github.com/sigp/lighthouse/pull/3790)
- `attestation_rewards`
- `block_rewards`.

This PR *only* implements the `attestation_rewards`.

The endpoints can be viewed in the Ethereum Beacon nodes API browser: 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).

---
- [x] `get_state`
- [x] Calculate *ideal rewards* with some logic from  `get_flag_index_deltas`
- [x] Calculate *actual rewards*  with some logic from `get_flag_index_deltas`
- [x] Code cleanup
- [x] Testing

common/eth2/src/lib.rs

@@ -1044,6 +1044,24 @@ impl BeaconNodeHttpClient {
         Ok(())
     }
 
+    /// `POST beacon/rewards/attestations`
+    pub async fn post_beacon_rewards_attestations(
+        &self,
+        attestations: &[ValidatorId],
+    ) -> 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("attestations");
+
+        self.post(path, &attestations).await?;
+
+        Ok(())
+    }
+
     /// `POST validator/contribution_and_proofs`
     pub async fn post_validator_contribution_and_proofs<T: EthSpec>(
         &self,