[PLUGIN][OAuth2] Start adding OAuth2 support with client registration

This hardcodes the user, and has some other issues, so it is not yet
complete.

We follow mastodon's spec for automatic client registration, available
at both `/api/v1/apps` and a more reasonable `/oauth/client`. This
accepts a JSON POST with the client info and returns JSON with a
`client_id` and a `client_secret`, to be used with `/oauth/authorize`
and `/oauth/token`. It also, seemingly, requires returning an `id`
with unclear purpose.

The `/oauth/token` endpoint doesn't currently return a `me` field.

plugins/OAuth2/Repository/AuthCode.php

@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types = 1);
+
+// {{{ License
+// This file is part of GNU social - https://www.gnu.org/software/social
+//
+// GNU social 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.
+//
+// GNU social 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 GNU social.  If not, see <http://www.gnu.org/licenses/>.
+// }}}
+
+/**
+ * OAuth2 implementation for GNU social
+ *
+ * @package   OAuth2
+ * @category  API
+ *
+ * @author    Diogo Peralta Cordeiro <mail@diogo.site>
+ * @author    Hugo Sales <hugo@hsal.es>
+ * @copyright 2022 Free Software Foundation, Inc http://www.fsf.org
+ * @license   https://www.gnu.org/licenses/agpl.html GNU AGPL v3 or later
+ */
+
+namespace Plugin\OAuth2\Repository;
+
+use App\Core\DB\DB;
+use League\OAuth2\Server\Entities\AuthCodeEntityInterface;
+use League\OAuth2\Server\Repositories\AuthCodeRepositoryInterface;
+use Plugin\OAuth2\Entity;
+
+class AuthCode implements AuthCodeRepositoryInterface
+{
+    public function persistNewAuthCode(AuthCodeEntityInterface $authCodeEntity)
+    {
+        DB::persist($authCodeEntity);
+    }
+
+    public function revokeAuthCode($codeId)
+    {
+        // Some logic to revoke the auth code in a database
+    }
+
+    public function isAuthCodeRevoked($codeId)
+    {
+        return false; // The auth code has not been revoked
+    }
+
+    public function getNewAuthCode()
+    {
+        return new Entity\AuthCode();
+    }
+}