Introduce the Windows lcow diff/snaphotter

Implements the Windows lcow differ/snapshotter responsible for managing
the creation and lifetime of lcow containers on Windows.

Signed-off-by: Justin Terry (VM) <juterry@microsoft.com>

runtime/v2/manager.go

@@ -30,10 +30,8 @@ import (
 	"github.com/containerd/containerd/metadata"
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/namespaces"
-	"github.com/containerd/containerd/platforms"
 	"github.com/containerd/containerd/plugin"
 	"github.com/containerd/containerd/runtime"
-	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
 func init() {
@@ -44,7 +42,7 @@ func init() {
 			plugin.MetadataPlugin,
 		},
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
-			ic.Meta.Platforms = []ocispec.Platform{platforms.DefaultSpec()}
+			ic.Meta.Platforms = supportedPlatforms()
 			if err := os.MkdirAll(ic.Root, 0711); err != nil {
 				return nil, err
 			}