Update to superstruct v0.4.1 (#2886)

## Proposed Changes

Update `superstruct` to bring in @realbigsean's fixes necessary for MEV-compatible private beacon block types (a la #2795).

The refactoring is due to another change in superstruct that allows partial getters to be auto-generated.

beacon_node/beacon_chain/src/execution_payload.rs

@@ -146,7 +146,7 @@ pub fn validate_execution_payload_for_gossip<T: BeaconChainTypes>(
     chain: &BeaconChain<T>,
 ) -> Result<(), BlockError<T::EthSpec>> {
     // Only apply this validation if this is a merge beacon block.
-    if let Some(execution_payload) = block.body().execution_payload() {
+    if let Ok(execution_payload) = block.body().execution_payload() {
         // This logic should match `is_execution_enabled`. We use only the execution block hash of
         // the parent here in order to avoid loading the parent state during gossip verification.
 
@@ -289,6 +289,7 @@ pub async fn prepare_execution_payload<T: BeaconChainTypes>(
                 .message()
                 .body()
                 .execution_payload()
+                .ok()
                 .map(|ep| ep.block_hash)
         };