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>

consensus/types/src/execution_payload.rs

@@ -152,3 +152,12 @@ impl<T: EthSpec> ForkVersionDeserialize for ExecutionPayload<T> {
         })
     }
 }
+
+impl<T: EthSpec> ExecutionPayload<T> {
+    pub fn fork_name(&self) -> ForkName {
+        match self {
+            ExecutionPayload::Merge(_) => ForkName::Merge,
+            ExecutionPayload::Capella(_) => ForkName::Capella,
+        }
+    }
+}