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,

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,
 };

common/eth2/src/lighthouse/attestation_rewards.rs

@@ -0,0 +1,42 @@
+use serde::{Deserialize, Serialize};
+
+// Details about the rewards paid for attestations
+// All rewards in GWei
+
+#[derive(Debug, PartialEq, Eq, Clone, Serialize, Deserialize)]
+pub struct IdealAttestationRewards {
+    // Validator's effective balance in gwei
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
+    pub effective_balance: u64,
+    // Ideal attester's reward for head vote in gwei
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
+    pub head: u64,
+    // Ideal attester's reward for target vote in gwei
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
+    pub target: u64,
+    // Ideal attester's reward for source vote in gwei
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
+    pub source: u64,
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Serialize, Deserialize)]
+pub struct TotalAttestationRewards {
+    // one entry for every validator based on their attestations in the epoch
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
+    pub validator_index: u64,
+    // attester's reward for head vote in gwei
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
+    pub head: u64,
+    // attester's reward for target vote in gwei
+    pub target: i64,
+    // attester's reward for source vote in gwei
+    pub source: i64,
+    // TBD attester's inclusion_delay reward in gwei (phase0 only)
+    // pub inclusion_delay: u64,
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Serialize, Deserialize)]
+pub struct StandardAttestationRewards {
+    pub ideal_rewards: Vec<IdealAttestationRewards>,
+    pub total_rewards: Vec<TotalAttestationRewards>,
+}

common/eth2/src/types.rs

@@ -270,11 +270,20 @@ pub struct FinalityCheckpointsData {
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(try_from = "&str")]
 pub enum ValidatorId {
     PublicKey(PublicKeyBytes),
     Index(u64),
 }
 
+impl TryFrom<&str> for ValidatorId {
+    type Error = String;
+
+    fn try_from(s: &str) -> Result<Self, Self::Error> {
+        Self::from_str(s)
+    }
+}
+
 impl FromStr for ValidatorId {
     type Err = String;