Add guild scheduled event endpoints (#3)

https://discord.com/developers/docs/resources/guild-scheduled-event

Author: SQKo

src/Discord/Endpoint.php

@@ -160,6 +160,13 @@ class Endpoint
     // GET
     public const GUILD_WEBHOOKS = self::GUILD.'/webhooks';
 
+    // GET, POST
+    public const GUILD_SCHEDULED_EVENTS = self::GUILD.'/scheduled-events';
+    // GET, PATCH, DELETE
+    public const GUILD_SCHEDULED_EVENT = self::GUILD_SCHEDULED_EVENTS.'/scheduled-events/:guild_scheduled_event_id';
+    // GET
+    public const GUILD_SCHEDULED_EVENT_USERS = self::GUILD.'/scheduled-events/:guild_scheduled_event_id/users';
+
     // GET, DELETE
     public const INVITE = 'invites/:code';