Block v3 endpoint (#4629)

## Issue Addressed

#4582

## Proposed Changes

Add a new v3 block fetching flow that can decide to return a Full OR Blinded payload

## Additional Info



Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/execution_layer/src/engine_api.rs

@@ -466,6 +466,11 @@ impl<T: EthSpec> From<GetPayloadResponse<T>>
     }
 }
 
+pub enum GetPayloadResponseType<E: EthSpec> {
+    Full(GetPayloadResponse<E>),
+    Blinded(GetPayloadResponse<E>),
+}
+
 impl<T: EthSpec> GetPayloadResponse<T> {
     pub fn execution_payload_ref(&self) -> ExecutionPayloadRef<T> {
         self.to_ref().into()