Include file fields in File source plugin if file_entity module is enabled

Problem/Motivation

File Entity → module makes file entity in Drupal 7 fieldable. Obviously, people will want to migrate some of this information stored in File fields into Drupal 9 website and save it in a Media entity, for example.

Currently, the Drupal\file\Plugin\migrate\source\d7\File source plugin is not aware of any potential fields, so they're not available in the processing pipeline.

Proposed resolution

1) Change the base class for File source plugin from DrupalSqlBase to FieldableEntity, that provides helper methods to retrieve entity values;
2) Add field values to the source row if file_entity module is present.

Remaining tasks

1) Verify language retrieval from entity_translation settings;
2) Add tests?
3) Review, commit.