Add config for allowing GC to clean unpacked layers up

This commit adds a flag through Pull API for allowing GC to clean layer contents
up after unpacking these contents completed.

This patch takes an approach to directly delete GC labels pointing to layers
from the manifest blob. This will result in other snapshotters cannot reuse
these contents on the next pull. But this patch mainly focuses on CRI use-cases
where single snapshotter is usually used throughout the node lifecycle so this
shouldn't be a matter.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

client.go

@@ -312,6 +312,11 @@ type RemoteContext struct {
 	// afterwards. Unpacking is required to run an image.
 	Unpack bool
 
+	// DiscardContent is a boolean flag to specify whether to allow GC to clean
+	// layers up from the content store after successfully unpacking these
+	// contents to the snapshotter.
+	DiscardContent bool
+
 	// UnpackOpts handles options to the unpack call.
 	UnpackOpts []UnpackOpt