Add CSS Inliner interface matching Emogrifier required methods

[MAILPOET-6428]

packages/php/email-editor/src/Engine/Renderer/interface-css-inliner.php

@@ -0,0 +1,35 @@
+<?php
+/**
+ * This file is part of the MailPoet Email Editor package.
+ *
+ * @package MailPoet\EmailEditor
+ */
+
+namespace MailPoet\EmailEditor\Engine\Renderer;
+
+interface Css_Inliner {
+	/**
+	 * Builds a new instance from the given HTML.
+	 *
+	 * @param string $unprocessed_html raw HTML, must be UTF-encoded, must not be empty.
+	 *
+	 * @return static
+	 */
+	public function from_html( string $unprocessed_html ): self;
+
+	/**
+	 * Inlines the given CSS into the existing HTML.
+	 *
+	 * @param string $css the CSS to inline, must be UTF-8-encoded.
+	 *
+	 * @return $this
+	 */
+	public function inline_css( string $css = '' ): self;
+
+	/**
+	 * Renders the normalized and processed HTML.
+	 *
+	 * @return string
+	 */
+	public function render(): string;
+}