Data source fields in CSV import should be overrideable by entity

[cbogart]

If you know some source identify for an entity imported via CSV file, you should be able to optionally specify it as the entity_source_id (with a custom entity_source_descriptor) as special CSV fields. Currently, entities are identified with their column and row in the CSV file, which only lets you trace provenance back to that file, rather than to the original source data.