Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/moby/buildkit/session/auth/auth.go

@@ -0,0 +1,27 @@
+package auth
+
+import (
+	"context"
+
+	"github.com/moby/buildkit/session"
+	"github.com/pkg/errors"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+func CredentialsFunc(ctx context.Context, c session.Caller) func(string) (string, string, error) {
+	return func(host string) (string, string, error) {
+		client := NewAuthClient(c.Conn())
+
+		resp, err := client.Credentials(ctx, &CredentialsRequest{
+			Host: host,
+		})
+		if err != nil {
+			if st, ok := status.FromError(errors.Cause(err)); ok && st.Code() == codes.Unimplemented {
+				return "", "", nil
+			}
+			return "", "", errors.WithStack(err)
+		}
+		return resp.Username, resp.Secret, nil
+	}
+}