Test database schema stability (#7669)

This PR implements some heuristics to check for breaking database changes. The goal is to prevent accidental changes to the database schema occurring without a version bump.

beacon_node/network/src/persisted_dht.rs

@@ -86,5 +86,9 @@ mod tests {
             .unwrap();
         let dht: PersistedDht = store.get_item(&DHT_DB_KEY).unwrap().unwrap();
         assert_eq!(dht.enrs, enrs);
+
+        // This hardcoded length check is for database schema compatibility. If the on-disk format
+        // of `PersistedDht` changes, we need a DB schema change.
+        assert_eq!(dht.as_store_bytes().len(), 136);
     }
 }