Reconstruct Payloads using Payload Bodies Methods (#4028)

## Issue Addressed

* #3895 

Co-authored-by: ethDreamer <37123614+ethDreamer@users.noreply.github.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/execution_layer/src/engine_api/json_structures.rs

@@ -3,7 +3,7 @@ use serde::{Deserialize, Serialize};
 use strum::EnumString;
 use superstruct::superstruct;
 use types::{
-    EthSpec, ExecutionBlockHash, FixedVector, Transaction, Unsigned, VariableList, Withdrawal,
+    EthSpec, ExecutionBlockHash, FixedVector, Transactions, Unsigned, VariableList, Withdrawal,
 };
 use types::{ExecutionPayload, ExecutionPayloadCapella, ExecutionPayloadMerge};
 
@@ -93,8 +93,7 @@ pub struct JsonExecutionPayload<T: EthSpec> {
     pub base_fee_per_gas: Uint256,
     pub block_hash: ExecutionBlockHash,
     #[serde(with = "ssz_types::serde_utils::list_of_hex_var_list")]
-    pub transactions:
-        VariableList<Transaction<T::MaxBytesPerTransaction>, T::MaxTransactionsPerPayload>,
+    pub transactions: Transactions<T>,
     #[superstruct(only(V2))]
     pub withdrawals: VariableList<JsonWithdrawal, T::MaxWithdrawalsPerPayload>,
 }
@@ -494,6 +493,30 @@ impl From<ForkchoiceUpdatedResponse> for JsonForkchoiceUpdatedV1Response {
     }
 }
 
+#[derive(Clone, Debug, Serialize, Deserialize)]
+#[serde(bound = "E: EthSpec")]
+pub struct JsonExecutionPayloadBodyV1<E: EthSpec> {
+    #[serde(with = "ssz_types::serde_utils::list_of_hex_var_list")]
+    pub transactions: Transactions<E>,
+    pub withdrawals: Option<VariableList<JsonWithdrawal, E::MaxWithdrawalsPerPayload>>,
+}
+
+impl<E: EthSpec> From<JsonExecutionPayloadBodyV1<E>> for ExecutionPayloadBodyV1<E> {
+    fn from(value: JsonExecutionPayloadBodyV1<E>) -> Self {
+        Self {
+            transactions: value.transactions,
+            withdrawals: value.withdrawals.map(|json_withdrawals| {
+                Withdrawals::<E>::from(
+                    json_withdrawals
+                        .into_iter()
+                        .map(Into::into)
+                        .collect::<Vec<_>>(),
+                )
+            }),
+        }
+    }
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct TransitionConfigurationV1 {