Add Feature to rewrite file urls

Problem/Motivation

A common use case in decoupled scenarious is to have a custom domain to access files from the frontend.

It would be a helpful addition to allow a per-consumer rewrite of the public and/or private files.

Proposed resolution

Internally Drupal uses public:// and private:// URIs which are later turned into absolute URLs via the FileUrlGenerator service.

By utilizing the file_url_alter() the URIs can be manually converted to absolute URLs.