Add unittests for RemoteClusterStateAttributesManager (opensearch-project#14427)

* Add unittests for RemoteClusterStateAttributesManager

Signed-off-by: Shivansh Arora <[email protected]>

Author: shiv0408

server/src/main/java/org/opensearch/gateway/remote/model/RemoteClusterStateCustoms.java

@@ -34,12 +34,12 @@
  */
 public class RemoteClusterStateCustoms extends AbstractRemoteWritableBlobEntity<Custom> {
     public static final String CLUSTER_STATE_CUSTOM = "cluster-state-custom";
+    public final ChecksumWritableBlobStoreFormat<ClusterState.Custom> clusterStateCustomsFormat;
 
     private long stateVersion;
     private final String customType;
     private ClusterState.Custom custom;
     private final NamedWriteableRegistry namedWriteableRegistry;
-    private final ChecksumWritableBlobStoreFormat<ClusterState.Custom> clusterStateCustomsFormat;
 
     public RemoteClusterStateCustoms(
         final ClusterState.Custom custom,