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>

consensus/types/src/payload.rs

@@ -979,3 +979,10 @@ impl<T: EthSpec> From<BlindedPayload<T>> for ExecutionPayloadHeader<T> {
         }
     }
 }
+
+/// The block production flow version to be used.
+pub enum BlockProductionVersion {
+    V3,
+    BlindedV2,
+    FullV2,
+}