Pass snapshotter opts during Pull

For remote snapshotter cases it's quite often there is need to pass extra info
from client (for instance - registry URL to query remote layer from, credentials, etc).

This commit slightly extends WithPullSnapshotter to pass extra labels to a snapshotter.

Signed-off-by: Maksym Pavlenko <makpav@amazon.com>

client.go

@@ -319,6 +319,9 @@ type RemoteContext struct {
 	// Snapshotter used for unpacking
 	Snapshotter string
 
+	// SnapshotterOpts are additional options to be passed to a snapshotter during pull
+	SnapshotterOpts []snapshots.Opt
+
 	// Labels to be applied to the created image
 	Labels map[string]string