Bucket: add prefix support for GCP

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

docs/spec/v1beta2/buckets.md

@@ -790,8 +790,8 @@ details and examples.
 `.spec.prefix` is an optional field to enable server-side filtering
 of files in the Bucket.
 
-**Note:** The server-side filtering works only with the `generic` and `aws`
-[provider](#provider) and is preferred over [`.spec.ignore`](#ignore)
+**Note:** The server-side filtering works only with the `generic`, `aws`
+and `gcp` [provider](#provider) and is preferred over [`.spec.ignore`](#ignore)
 as a more efficient way of excluding files. 
 
 ### Ignore

pkg/gcp/gcp.go

@@ -166,7 +166,9 @@ func (c *GCSClient) FGetObject(ctx context.Context, bucketName, objectName, loca
 // If the underlying client or the visit callback returns an error,
 // it returns early.
 func (c *GCSClient) VisitObjects(ctx context.Context, bucketName string, prefix string, visit func(path, etag string) error) error {
-	items := c.Client.Bucket(bucketName).Objects(ctx, nil)
+	items := c.Client.Bucket(bucketName).Objects(ctx, &gcpstorage.Query{
+		Prefix: prefix,
+	})
 	for {
 		object, err := items.Next()
 		if err == IteratorDone {