Implement expected withdrawals endpoint (#4390)

## Issue Addressed

[#4029](https://github.com/sigp/lighthouse/issues/4029)

## Proposed Changes

implement expected_withdrawals HTTP API per the spec 

https://github.com/ethereum/beacon-APIs/pull/304

## Additional Info

beacon_node/http_api/tests/tests.rs

@@ -28,6 +28,7 @@ use sensitive_url::SensitiveUrl;
 use slot_clock::SlotClock;
 use state_processing::per_block_processing::get_expected_withdrawals;
 use state_processing::per_slot_processing;
+use state_processing::state_advance::partial_state_advance;
 use std::convert::TryInto;
 use std::sync::Arc;
 use tokio::time::Duration;
@@ -4341,6 +4342,72 @@ impl ApiTester {
         self
     }
 
+    pub async fn test_get_expected_withdrawals_invalid_state(self) -> Self {
+        let state_id = CoreStateId::Root(Hash256::zero());
+
+        let result = self.client.get_expected_withdrawals(&state_id).await;
+
+        match result {
+            Err(e) => {
+                assert_eq!(e.status().unwrap(), 404);
+            }
+            _ => panic!("query did not fail correctly"),
+        }
+
+        self
+    }
+
+    pub async fn test_get_expected_withdrawals_capella(self) -> Self {
+        let slot = self.chain.slot().unwrap();
+        let state_id = CoreStateId::Slot(slot);
+
+        // calculate the expected withdrawals
+        let (mut state, _, _) = StateId(state_id).state(&self.chain).unwrap();
+        let proposal_slot = state.slot() + 1;
+        let proposal_epoch = proposal_slot.epoch(E::slots_per_epoch());
+        let (state_root, _, _) = StateId(state_id).root(&self.chain).unwrap();
+        if proposal_epoch != state.current_epoch() {
+            let _ = partial_state_advance(
+                &mut state,
+                Some(state_root),
+                proposal_slot,
+                &self.chain.spec,
+            );
+        }
+        let expected_withdrawals = get_expected_withdrawals(&state, &self.chain.spec).unwrap();
+
+        // fetch expected withdrawals from the client
+        let result = self.client.get_expected_withdrawals(&state_id).await;
+        match result {
+            Ok(withdrawal_response) => {
+                assert_eq!(withdrawal_response.execution_optimistic, Some(false));
+                assert_eq!(withdrawal_response.finalized, Some(false));
+                assert_eq!(withdrawal_response.data, expected_withdrawals.to_vec());
+            }
+            Err(e) => {
+                println!("{:?}", e);
+                panic!("query failed incorrectly");
+            }
+        }
+
+        self
+    }
+
+    pub async fn test_get_expected_withdrawals_pre_capella(self) -> Self {
+        let state_id = CoreStateId::Head;
+
+        let result = self.client.get_expected_withdrawals(&state_id).await;
+
+        match result {
+            Err(e) => {
+                assert_eq!(e.status().unwrap(), 400);
+            }
+            _ => panic!("query did not fail correctly"),
+        }
+
+        self
+    }
+
     pub async fn test_get_events_altair(self) -> Self {
         let topics = vec![EventTopic::ContributionAndProof];
         let mut events_future = self
@@ -5123,3 +5190,37 @@ async fn optimistic_responses() {
         .test_check_optimistic_responses()
         .await;
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn expected_withdrawals_invalid_pre_capella() {
+    let mut config = ApiTesterConfig::default();
+    config.spec.altair_fork_epoch = Some(Epoch::new(0));
+    ApiTester::new_from_config(config)
+        .await
+        .test_get_expected_withdrawals_pre_capella()
+        .await;
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn expected_withdrawals_invalid_state() {
+    let mut config = ApiTesterConfig::default();
+    config.spec.altair_fork_epoch = Some(Epoch::new(0));
+    config.spec.bellatrix_fork_epoch = Some(Epoch::new(0));
+    config.spec.capella_fork_epoch = Some(Epoch::new(0));
+    ApiTester::new_from_config(config)
+        .await
+        .test_get_expected_withdrawals_invalid_state()
+        .await;
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn expected_withdrawals_valid_capella() {
+    let mut config = ApiTesterConfig::default();
+    config.spec.altair_fork_epoch = Some(Epoch::new(0));
+    config.spec.bellatrix_fork_epoch = Some(Epoch::new(0));
+    config.spec.capella_fork_epoch = Some(Epoch::new(0));
+    ApiTester::new_from_config(config)
+        .await
+        .test_get_expected_withdrawals_capella()
+        .await;
+}