Implement sync_committee_rewards API (per-validator reward) (#3903)

## Issue Addressed

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

## Proposed Changes

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

```
{
  "execution_optimistic": false,
  "finalized": false,
  "data": [
    {
      "validator_index": "0",
      "reward": "2000"
    }
  ]
}
```

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

common/eth2/src/lib.rs

@@ -1026,6 +1026,24 @@ impl BeaconNodeHttpClient {
             .transpose()
     }
 
+    /// `POST beacon/rewards/sync_committee`
+    pub async fn post_beacon_rewards_sync_committee(
+        &self,
+        rewards: &[Option<Vec<lighthouse::SyncCommitteeReward>>],
+    ) -> 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("sync_committee");
+
+        self.post(path, &rewards).await?;
+
+        Ok(())
+    }
+
     /// `POST validator/contribution_and_proofs`
     pub async fn post_validator_contribution_and_proofs<T: EthSpec>(
         &self,

common/eth2/src/lighthouse.rs

@@ -3,6 +3,7 @@
 mod attestation_performance;
 mod block_packing_efficiency;
 mod block_rewards;
+mod sync_committee_rewards;
 
 use crate::{
     ok_or_error,
@@ -27,6 +28,7 @@ pub use block_packing_efficiency::{
 };
 pub use block_rewards::{AttestationRewards, BlockReward, BlockRewardMeta, BlockRewardsQuery};
 pub use lighthouse_network::{types::SyncState, PeerInfo};
+pub use sync_committee_rewards::SyncCommitteeReward;
 
 // Define "legacy" implementations of `Option<T>` which use four bytes for encoding the union
 // selector.

common/eth2/src/lighthouse/sync_committee_rewards.rs

@@ -0,0 +1,12 @@
+use serde::{Deserialize, Serialize};
+
+// Details about the rewards paid to sync committee members for attesting headers
+// All rewards in GWei
+
+#[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
+pub struct SyncCommitteeReward {
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
+    pub validator_index: u64,
+    // sync committee reward in gwei for the validator
+    pub reward: i64,
+}