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

common/eth2/src/lib.rs

@@ -1261,6 +1261,23 @@ impl BeaconNodeHttpClient {
         Ok(())
     }
 
+    // GET builder/states/{state_id}/expected_withdrawals
+    pub async fn get_expected_withdrawals(
+        &self,
+        state_id: &StateId,
+    ) -> Result<ExecutionOptimisticFinalizedResponse<Vec<Withdrawal>>, Error> {
+        let mut path = self.eth_path(V1)?;
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("builder")
+            .push("states")
+            .push(&state_id.to_string())
+            .push("expected_withdrawals");
+
+        self.get(path).await
+    }
+
     /// `POST validator/contribution_and_proofs`
     pub async fn post_validator_contribution_and_proofs<T: EthSpec>(
         &self,

common/eth2/src/types.rs

@@ -581,6 +581,11 @@ pub struct SyncingData {
     pub sync_distance: Slot,
 }
 
+#[derive(Serialize, Deserialize)]
+pub struct ExpectedWithdrawalsQuery {
+    pub proposal_slot: Option<Slot>,
+}
+
 #[derive(Clone, PartialEq, Debug, Deserialize)]
 #[serde(try_from = "String", bound = "T: FromStr")]
 pub struct QueryVec<T: FromStr> {