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/lighthouse.rs

@@ -1,6 +1,7 @@
 //! This module contains endpoints that are non-standard and only available on Lighthouse servers.
 
 mod attestation_performance;
+pub mod attestation_rewards;
 mod block_packing_efficiency;
 mod block_rewards;
 mod sync_committee_rewards;
@@ -23,6 +24,7 @@ use store::{AnchorInfo, Split, StoreConfig};
 pub use attestation_performance::{
     AttestationPerformance, AttestationPerformanceQuery, AttestationPerformanceStatistics,
 };
+pub use attestation_rewards::StandardAttestationRewards;
 pub use block_packing_efficiency::{
     BlockPackingEfficiency, BlockPackingEfficiencyQuery, ProposerInfo, UniqueAttestation,
 };