Port the hook_file_url_alter() implementation from #3577151 to 3.1.x

Problem/Motivation

#3577151: Implement hook_file_url_alter() to support linking to files that do not exist yet locally adds a hook_file_url_alter() implementation that points a public file which is not present locally at the origin site, together with DownloadManager::rewriteFileUrl() behind it. In #3577151-3: Implement hook_file_url_alter() to support linking to files that do not exist yet locally that issue was retargeted from 3.1.x-dev to 4.0.x-dev.

That leaves the change out of reach for every site that cannot run 4.0.x:

• 4.0.x declares core_version_requirement: ^11.3 || ^12.0
• 3.1.x declares core_version_requirement: ^10.3 || ^11

So a site on Drupal 10 can only install 3.1.x, and on 3.1.x hook_file_url_alter() is not implemented at all.

The gap shows up for files whose URL is rendered into markup and then fetched without the request reaching this site in a way the module's request subscriber can act on. Non-image files such as PDFs, video, or packaged HTML (SCORM) are the common case, which is the same scenario #3577151: Implement hook_file_url_alter() to support linking to files that do not exist yet locally describes.

Proposed resolution

Port the same change to 3.1.x, leaving out the parts 3.1.x cannot use:

• Add rewriteFileUrl() to DownloadManagerInterface and implement it in DownloadManager, identical to #3577151: Implement hook_file_url_alter() to support linking to files that do not exist yet locally.
• Implement the hook procedurally in stage_file_proxy.module instead of as an OOP hook. 3.1.x supports ^10.3, and Drupal 10 ships no Drupal\Core\Hook\Attribute\Hook class and no attribute-hook discovery, so a #[Hook] implementation would be installed but would silently never run there. A procedural implementation runs across the whole supported range.

Image style derivatives (public://styles/) are deliberately left alone, since those are already handled by the module's own image download controller.

Remaining tasks

• Review.
• Decide whether this should wait for #3577151: Implement hook_file_url_alter() to support linking to files that do not exist yet locally to land in 4.0.x first so the two branches stay in step. That merge request is still open at the time of writing.

User interface changes

None.

API changes

DownloadManagerInterface gains one method, rewriteFileUrl(). DownloadManager itself is @internal and final, but the interface is not, so a site providing its own implementation of the interface would need to add the method.

Data model changes

None.