Provide central files for common composer configuration

The following sections in composer.json files are almost always identical for all projects.

• config with all the settings, including allowed plugins and audit ignore lists.
• extra.drupal-scaffold
• extra.installer-types
• extra.installer-paths
• extra.drupal-libraries

Instead of maintaining those settings per project, let's manage them in separate files - similar to the patches.json file.

But we need to test, if that really also works for fresh installations, where nothing exists other than the composer.json file. Does it still load all the necessary hooks even if the composer plugin that does the work, doesn't exist yet? Should be working as it does for composer patches as well.