Implements #3413902 - Mega Refactoring & General Improvements

The following main changes (enchantments, fixes, etc.) are being implemented:

• Implementation of configurations Settings yml file & Schema;
• Implementation of dynamic configurations for Media Types & Fields;
• Implementation of Drupal Queue API, via "Media Library Importer Queue", for properly managing Files imports and Media creation;
• Implementation of MediaLibraryImporterService, and relocation of helper methods into it, including public methods to trigger the generation & processing of Media Import Queue;
• Implementation and documentation of Drush command for triggering Files import and Media creations;
• Adjustment and integration of Documentation, both in hook_help and in the Readme.md file, with additional instruction (code gist) on how to generate a cron hook/job for automatically process Files import and Media creations;
• Extended and complete PHP & Drupal coding standards fixes and alignments;

@salihcenap please consider adding me (@itamair) as official co-maintainer of the Media Library Importer module.