Generate srcset for remote images from allowed 3rd parties

This is omitting something crucial surfaced in research that happened in #3535453:

since \Drupal\image\Controller\ImageStyleDownloadController::deliver() does NOT support "shipped files" (aka files shipped as part of a module or theme), even though the logic in \Drupal\image\Entity\ImageStyle::buildUri() suggests it does (see the else branch).

— yours truly at #3535453-33: Create an Image SDC that can be included by other SDCs → .

Also: the issue summary implies this might be limited to SDC-powered components. But … why? 🤔 Why shouldn't this then also work for other remote images, including those specified in code components?