Make sure webhook messages can be versionised later.

app/Helpers/Webhook/SignatureGeneratorInterface.php

@@ -0,0 +1,44 @@
+<?php
+/*
+ * SignatureGeneratorInterface.php
+ * Copyright (c) 2020 james@firefly-iii.org
+ *
+ * This file is part of Firefly III (https://github.com/firefly-iii).
+ *
+ * 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 <https://www.gnu.org/licenses/>.
+ */
+
+namespace FireflyIII\Helpers\Webhook;
+
+use FireflyIII\Models\WebhookMessage;
+
+/**
+ * Interface SignatureGeneratorInterface
+ */
+interface SignatureGeneratorInterface
+{
+    /**
+     * Return the version of this signature generator.
+     *
+     * @return int
+     */
+    public function getVersion(): int;
+
+    /**
+     * @param WebhookMessage $message
+     *
+     * @return string
+     */
+    public function generate(WebhookMessage $message): string;
+}