feat: create permission decorator

This gathers the permission a user needs to hold to access a resource for the PermissionsGuard.

See https://docs.nestjs.com/guards#setting-roles-per-handler

Signed-off-by: Philip Molares <philip.molares@udo.edu>

src/permissions/permissions.decorator.ts

@@ -0,0 +1,17 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { CustomDecorator, SetMetadata } from '@nestjs/common';
+
+import { Permission } from './permissions.enum';
+
+/**
+ * This decorator gathers the {@link Permission Permission} a user must hold for the {@link PermissionsGuard}
+ * @param permissions - an array of permissions. In practice this should always contain exactly one {@link Permission}
+ * @constructor
+ */
+// eslint-disable-next-line func-style,@typescript-eslint/naming-convention
+export const Permissions = (...permissions: Permission[]): CustomDecorator =>
+  SetMetadata('permissions', permissions);