Add support for syncing replica modifications (#11104)

when bidirectional replication is set up.

If ReplicaModifications is enabled in the replication
configuration, sync metadata updates to source if
replication rules are met. By default, if this
configuration is unset, MinIO automatically sync's
metadata updates on replica back to the source.

docs/bucket/replication/README.md

@@ -132,6 +132,11 @@ The replication configuration can now be added to the source bucket by applying
       "Destination": {
         "Bucket": "arn:aws:s3:::destbucket",
         "StorageClass": "STANDARD"
+      },
+      "SourceSelectionCriteria": {
+        "ReplicaModifications": {
+          "Status": "Enabled"
+        }
       }
     }
   ]
@@ -150,6 +155,11 @@ To perform bi-directional replication, repeat the above process on the target si
 
 ![head](https://raw.githubusercontent.com/minio/minio/master/docs/bucket/replication/HEAD_bucket_replication.png)
 
+## Replica Modification sync
+If bi-directional replication is set up between two clusters, any metadata update on the REPLICA object is by default reflected back in the source object when `ReplicaModifications` status in the `SourceSelectionCriteria` is `Enabled`. In MinIO, this is enabled by default. If a metadata update is performed on the "REPLICA" object, its `X-Amz-Replication-Status` will change from `PENDING` to `COMPLETE` or `FAILED`, and the source object version will show `X-Amz-Replication-Status` of `REPLICA` once the replication operation is complete.
+
+The replication configuration in use on a bucket can be viewed using the `mc replicate export alias/bucket` command.
+
 ## MinIO Extension
 ### Replicating Deletes