add IVirtualFeed interface for plugins implementing virtual feeds

classes/ivirtualfeed.php

@@ -0,0 +1,11 @@
+<?php
+interface IVirtualFeed {
+	function get_unread(int $feed_id) : int;
+	function get_total(int $feed_id) : int;
+	/**
+	 * @param int $feed_id
+	 * @param array<string,int|string|bool> $options
+	 * @return array<int,int|string>
+	 */
+	function get_headlines(int $feed_id, array $options) : array;
+}