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>

beacon_node/http_api/src/lib.rs

@@ -15,6 +15,7 @@ mod database;
 mod metrics;
 mod proposer_duties;
 mod publish_blocks;
+mod standard_block_rewards;
 mod state_id;
 mod sync_committee_rewards;
 mod sync_committees;
@@ -1700,6 +1701,27 @@ pub fn serve<T: BeaconChainTypes>(
             },
         );
 
+    let beacon_rewards_path = eth_v1
+        .and(warp::path("beacon"))
+        .and(warp::path("rewards"))
+        .and(chain_filter.clone());
+
+    // GET beacon/rewards/blocks/{block_id}
+    let get_beacon_rewards_blocks = beacon_rewards_path
+        .clone()
+        .and(warp::path("blocks"))
+        .and(block_id_or_err)
+        .and(warp::path::end())
+        .and_then(|chain: Arc<BeaconChain<T>>, block_id: BlockId| {
+            blocking_json_task(move || {
+                let (rewards, execution_optimistic) =
+                    standard_block_rewards::compute_beacon_block_rewards(chain, block_id)?;
+                Ok(rewards)
+                    .map(api_types::GenericResponse::from)
+                    .map(|resp| resp.add_execution_optimistic(execution_optimistic))
+            })
+        });
+
     /*
      * beacon/rewards
      */
@@ -3433,6 +3455,7 @@ pub fn serve<T: BeaconChainTypes>(
                 .or(get_beacon_pool_proposer_slashings.boxed())
                 .or(get_beacon_pool_voluntary_exits.boxed())
                 .or(get_beacon_deposit_snapshot.boxed())
+                .or(get_beacon_rewards_blocks.boxed())
                 .or(get_config_fork_schedule.boxed())
                 .or(get_config_spec.boxed())
                 .or(get_config_deposit_contract.boxed())

beacon_node/http_api/src/standard_block_rewards.rs

@@ -0,0 +1,27 @@
+use crate::sync_committee_rewards::get_state_before_applying_block;
+use crate::BlockId;
+use crate::ExecutionOptimistic;
+use beacon_chain::{BeaconChain, BeaconChainTypes};
+use eth2::lighthouse::StandardBlockReward;
+use std::sync::Arc;
+use warp_utils::reject::beacon_chain_error;
+//// The difference between block_rewards and beacon_block_rewards is the later returns block
+//// reward format that satisfies beacon-api specs
+pub fn compute_beacon_block_rewards<T: BeaconChainTypes>(
+    chain: Arc<BeaconChain<T>>,
+    block_id: BlockId,
+) -> Result<(StandardBlockReward, ExecutionOptimistic), warp::Rejection> {
+    let (block, execution_optimistic) = block_id.blinded_block(&chain)?;
+
+    let block_ref = block.message();
+
+    let block_root = block.canonical_root();
+
+    let mut state = get_state_before_applying_block(chain.clone(), &block)?;
+
+    let rewards = chain
+        .compute_beacon_block_reward(block_ref, block_root, &mut state)
+        .map_err(beacon_chain_error)?;
+
+    Ok((rewards, execution_optimistic))
+}

beacon_node/http_api/src/sync_committee_rewards.rs

@@ -47,7 +47,7 @@ pub fn compute_sync_committee_rewards<T: BeaconChainTypes>(
     Ok((data, execution_optimistic))
 }
 
-fn get_state_before_applying_block<T: BeaconChainTypes>(
+pub fn get_state_before_applying_block<T: BeaconChainTypes>(
     chain: Arc<BeaconChain<T>>,
     block: &SignedBlindedBeaconBlock<T::EthSpec>,
 ) -> Result<BeaconState<T::EthSpec>, warp::reject::Rejection> {