Add leaf_columns argument to ArrowReader::get_record_reader_by_columns

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

ArrowReader::get_schema_by_columns has a leaf_columns argument that switches between selecting between the indexes of leaf columns, and the indexes of the root schema element.

This is helpful because not only does it highlight to the end-user that there is a distinction, but depending on the use-case the user might want either behaviour.

Describe the solution you'd like

Add a leaf_columns field to ArrowReader::get_record_reader_by_columns

Describe alternatives you've considered

We could not do this

Additional context

apache/datafusion#2453