Add getter for file

Problem/Motivation

Building the UI to edit the column aliases requires the file object as it is after setupFile(). That is a protect method but fortunately it is called from the public method initializeIterator(). So that can be used. Unfortunately, that is not the case. It does not work because CSV::initializeIterator() doesn't always return the file object. In the case when the source plugin uses yield it returns a Generator.

See #2942373: Add UI to alter column_names for CSV source files

Proposed resolution

Add a getter for $file

Remaining tasks

Patch
review