New user backend interface to check if a user can confirm passwords

On some backends (saml for example) users can't revalidate their
passwords as Nextcloud has no way to do verify it.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

lib/public/User/Backend/IPasswordConfirmationBackend.php

@@ -0,0 +1,36 @@
+<?php
+declare(strict_types=1);
+/**
+ * @copyright Copyright (c) 2018, Roeland Jago Douma <roeland@famdouma.nl>
+ *
+ * @author Roeland Jago Douma <roeland@famdouma.nl>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+namespace OCP\User\Backend;
+
+/**
+ * @since 15.0.0
+ */
+interface IPasswordConfirmationBackend {
+
+	/**
+	 * @since 15.0.0
+	 */
+	public function canConfirmPassword(string $uid): bool;
+}