api: Introduce metadata update APIs to update only metadata (#11962)

Current implementation heavily relies on readAllFileInfo
but with the advent of xl.meta inlined with data, we cannot
easily avoid reading data when we are only interested is
updating metadata, this leads to invariably write
amplification during metadata updates, repeatedly reading
data when we are only interested in updating metadata.

This PR ensures that we implement a metadata only update
API at storage layer, that handles updates to metadata alone
for any given version - given the version is valid and
present.

This helps reduce the chattiness for following calls..

- PutObjectTags
- DeleteObjectTags
- PutObjectLegalHold
- PutObjectRetention
- ReplicateObject (updates metadata on replication status)

cmd/storage-rest-client.go

@@ -360,6 +360,21 @@ func (client *storageRESTClient) WriteMetadata(ctx context.Context, volume, path
 	return err
 }
 
+func (client *storageRESTClient) UpdateMetadata(ctx context.Context, volume, path string, fi FileInfo) error {
+	values := make(url.Values)
+	values.Set(storageRESTVolume, volume)
+	values.Set(storageRESTFilePath, path)
+
+	var reader bytes.Buffer
+	if err := msgp.Encode(&reader, &fi); err != nil {
+		return err
+	}
+
+	respBody, err := client.call(ctx, storageRESTMethodUpdateMetadata, values, &reader, -1)
+	defer http.DrainBody(respBody)
+	return err
+}
+
 func (client *storageRESTClient) DeleteVersion(ctx context.Context, volume, path string, fi FileInfo, forceDelMarker bool) error {
 	values := make(url.Values)
 	values.Set(storageRESTVolume, volume)