Add service for resolving files

I've done some testing and reviewing the code this week.

Latest changes:

• A BC break is avoided: when subclasses override the File target methods getFileName(), getContent() or writeData() a fallback to the old implementation will be followed. If we would leave out the fallback, then these methods would no longer get called if subclasses overrode them. And that possible would break workflows. The methods are still deprecated, though. So the fallback won't be there forever.
• A change record for this issue has been drafted.

Findings so far on reviewing/testing the code:

• When downloading a file without no extension, that file gets saved to file system - and then importing the entity fails because of validation errors, while previously the download was aborted - and the entity got imported. I plan to address this in #3076946: Failed to validate remote image with no file extension, which I already have worked on.
• With FileResolver, other protocols than "http" or "https" are no longer supported. In the previous implementation other protocols didn't work out of the box, but you could do so by adding the following to settings.php:
  

  $settings['http_client_config']['curl'] = [
    CURLOPT_PROTOCOLS => CURLPROTO_HTTP | CURLPROTO_HTTPS | CURLPROTO_FILE | CURLPROTO_FTP,
  ];
  

  I did not manage to get a successful import with a file specified with either file:// or ftp://, so I'm going to assume it is a far fetched edge case. We could re-add support for other protocols once users really need it.

I scheduled merging the code!