Deneb review common/eth2 (#4698)

* Update comments and small cleanup.

* Deserialize into `SsePayloadAttributesV3` for Deneb fork. Update `SignedBlockContents::blobs_cloned` to return blobs for `BlindedBlockAndBlobSidecars`.

* Improve code readability and error handling when converting blinded block into full block.

beacon_node/http_api/src/lib.rs

@@ -1729,7 +1729,7 @@ pub fn serve<T: BeaconChainTypes>(
         );
 
     /*
-     * beacon/blobs
+     * beacon/blob_sidecars
      */
 
     // GET beacon/blob_sidecars/{block_id}