Remote URL Support

• We should use Guzzle HTTP library instead of file_get_contents(), see https://www.drupal.org/node/1862446
• file_unmanaged_save_data() is deprecated too, see https://www.drupal.org/node/3006851
• We should be saving to the temp file system, not public

currently i am working on it ............

Please find the updated patch ......

Actually this code should be called from PDFPreviewGenerator::createPDFPreview(). The existing filemtime() checks in PDFPreviewGenerator::getPDFPreview() will be problematic. I guess we could do something like this:

  public function getPDFPreview(File $file) {
    $destination_uri = $this->getDestinationURI($file);
    // Check if a preview already exists.
    if (file_exists($destination_uri)) {
      if (UrlHelper::isExternal($file->getFileUri())) {
        return $destination_uri;
      }
      // Do the other mtime stuff
      // ...
    }
    if ($this->createPDFPreview($file, $destination_uri)) {
      return $destination_uri;
    }
  }

However that means we'll never generate an updated preview if the source PDF changes. Not sure how much we can do about that as I really don't want to download the PDF every time the preview is requested.

We also need to extend PDFPreviewGeneratorTest::testGetDestinationUri() to handle remote stream wrappers.