Define htmlToPdf in the Hook

This method was always required because it was called by the reporting
module assuming that our own implementation of the pdfexport module was
the one chosen.

Because this behavior exists and our pdfexport module is the only
implementation of this hook this is not a breaking change.

Author: TheSyscall

library/Icinga/Application/Hook/PdfexportHook.php

@@ -57,4 +57,13 @@ abstract public function isSupported();
      * @return never
      */
     abstract public function streamPdfFromHtml($html, $filename);
+
+    /**
+     * Render the specified HTML to PDF and return the PDF document as a string
+     *
+     * @param ValidHtml $html The HTML to render to PDF
+     *
+     * @return string
+     */
+    abstract public function htmlToPdf($html);
 }