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/src/builder_states.rs

@@ -0,0 +1,72 @@
+use crate::StateId;
+use beacon_chain::{BeaconChain, BeaconChainTypes};
+use safe_arith::SafeArith;
+use state_processing::per_block_processing::get_expected_withdrawals;
+use state_processing::state_advance::partial_state_advance;
+use std::sync::Arc;
+use types::{BeaconState, EthSpec, ForkName, Slot, Withdrawals};
+
+const MAX_EPOCH_LOOKAHEAD: u64 = 2;
+
+/// Get the withdrawals computed from the specified state, that will be included in the block
+/// that gets built on the specified state.
+pub fn get_next_withdrawals<T: BeaconChainTypes>(
+    chain: &Arc<BeaconChain<T>>,
+    mut state: BeaconState<T::EthSpec>,
+    state_id: StateId,
+    proposal_slot: Slot,
+) -> Result<Withdrawals<T::EthSpec>, warp::Rejection> {
+    get_next_withdrawals_sanity_checks(chain, &state, proposal_slot)?;
+
+    // advance the state to the epoch of the proposal slot.
+    let proposal_epoch = proposal_slot.epoch(T::EthSpec::slots_per_epoch());
+    let (state_root, _, _) = state_id.root(chain)?;
+    if proposal_epoch != state.current_epoch() {
+        if let Err(e) =
+            partial_state_advance(&mut state, Some(state_root), proposal_slot, &chain.spec)
+        {
+            return Err(warp_utils::reject::custom_server_error(format!(
+                "failed to advance to the epoch of the proposal slot: {:?}",
+                e
+            )));
+        }
+    }
+
+    match get_expected_withdrawals(&state, &chain.spec) {
+        Ok(withdrawals) => Ok(withdrawals),
+        Err(e) => Err(warp_utils::reject::custom_server_error(format!(
+            "failed to get expected withdrawal: {:?}",
+            e
+        ))),
+    }
+}
+
+fn get_next_withdrawals_sanity_checks<T: BeaconChainTypes>(
+    chain: &BeaconChain<T>,
+    state: &BeaconState<T::EthSpec>,
+    proposal_slot: Slot,
+) -> Result<(), warp::Rejection> {
+    if proposal_slot <= state.slot() {
+        return Err(warp_utils::reject::custom_bad_request(
+            "proposal slot must be greater than the pre-state slot".to_string(),
+        ));
+    }
+
+    let fork = chain.spec.fork_name_at_slot::<T::EthSpec>(proposal_slot);
+    if let ForkName::Base | ForkName::Altair | ForkName::Merge = fork {
+        return Err(warp_utils::reject::custom_bad_request(
+            "the specified state is a pre-capella state.".to_string(),
+        ));
+    }
+
+    let look_ahead_limit = MAX_EPOCH_LOOKAHEAD
+        .safe_mul(T::EthSpec::slots_per_epoch())
+        .map_err(warp_utils::reject::arith_error)?;
+    if proposal_slot >= state.slot() + look_ahead_limit {
+        return Err(warp_utils::reject::custom_bad_request(format!(
+            "proposal slot is greater than or equal to the look ahead limit: {look_ahead_limit}"
+        )));
+    }
+
+    Ok(())
+}