New directory for temporary, http downloaded files

Previously, files downloaded by the HTTP fetcher were saved to "temporary://", causing issues if the temporary directory changed or was emptied while an import was still running. Instead, http downloaded files are saved to private://feeds/in_progress/[feed ID] (or public://feeds/in_progress/[feed ID] in case no private file system is configured).

This directory is configurable:

$config['feeds.settings']['in_progress_dir'] = 'private://feeds/in_progress';

No UI exists to change it because the default should be good in most cases.

There also has been a new service added to interact with this directory: feeds.file_system.in_progress