Op pool and gossip for BLS to execution changes (#3726)

beacon_node/beacon_chain/src/builder.rs

@@ -780,6 +780,8 @@ where
             observed_voluntary_exits: <_>::default(),
             observed_proposer_slashings: <_>::default(),
             observed_attester_slashings: <_>::default(),
+            #[cfg(feature = "withdrawals-processing")]
+            observed_bls_to_execution_changes: <_>::default(),
             eth1_chain: self.eth1_chain,
             execution_layer: self.execution_layer,
             genesis_validators_root,