Add capability for snapshotters to declare support for UID remapping

This allows user namespace support to progress, either by allowing
snapshotters to deal with ownership, or falling back to containerd doing
a recursive chown.

In the future, when snapshotters implement idmap mounts, they should
report the "remap-ids" capability.

Co-authored-by: Rodrigo Campos <rodrigoca@microsoft.com>
Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>
Signed-off-by: David Leadbeater <dgl@dgl.cx>

client.go

@@ -866,3 +866,21 @@ func toPlatforms(pt []*apitypes.Platform) []ocispec.Platform {
 	}
 	return platforms
 }
+
+// GetSnapshotterCapabilities returns the capabilities of a snapshotter.
+func (c *Client) GetSnapshotterCapabilities(ctx context.Context, snapshotterName string) ([]string, error) {
+	filters := []string{fmt.Sprintf("type==%s, id==%s", plugin.SnapshotPlugin, snapshotterName)}
+	in := c.IntrospectionService()
+
+	resp, err := in.Plugins(ctx, filters)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(resp.Plugins) <= 0 {
+		return nil, fmt.Errorf("inspection service could not find snapshotter %s plugin", snapshotterName)
+	}
+
+	sn := resp.Plugins[0]
+	return sn.Capabilities, nil
+}