Fix column selection read_parquet benchmarks

[vuule]

Description

Helper function get_col_names in the Parquet reader benchmarks throws with nested columns. It should instead just ignore the children columns and return the top-level colum names.
Also renamed the function to better reflect what it does.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vuule]

/merge