Allow absolute URLs to be generated using the editor_file_reference filter

Believe this will need sub-maintainer approval for such a change, but could this functionality also live in a contrib module?

If approved will need an upgrade path and test coverage.

Tried to get ahold of sub-maintainer but they couldn't look, so bumping to framework manager.

The issue summary doesn't explain the problem with relative URLs, why is this a problem?

Additionally, Drupal sites can be run from multiple domains, using domain module and similar. Relative URLs allow this to work, I don't think we're able to cache text formats by arbitrary cache contexts so it would probably break that.

Also if we were to add this, I think it should be its own filter doing this generally for all site-relative links, not within the file reference filter.

For example https://www.drupal.org/project/rel_to_abs → provides this in contrib.