Download checkpoint blobs during checkpoint sync (#5252)

* MVP implementation (untested)

* update store checkpoint sync test

* update cli help

* Merge pull request #5253 from realbigsean/checkpoint-blobs-sean

Checkpoint blobs sean

* Warn only if blobs are missing from server

* Merge remote-tracking branch 'origin/unstable' into checkpoint-blobs

* Verify checkpoint blobs

* Move blob verification earlier

consensus/types/src/beacon_block_body.rs

@@ -176,6 +176,12 @@ impl<'a, T: EthSpec, Payload: AbstractExecPayload<T>> BeaconBlockBodyRef<'a, T,
             }
         }
     }
+
+    /// Return `true` if this block body has a non-zero number of blobs.
+    pub fn has_blobs(self) -> bool {
+        self.blob_kzg_commitments()
+            .map_or(false, |blobs| !blobs.is_empty())
+    }
 }
 
 impl<'a, T: EthSpec, Payload: AbstractExecPayload<T>> BeaconBlockBodyRef<'a, T, Payload> {