Function create_initial_plan 

fn create_initial_plan(
    file_name: &str,
    extensions: Option<Arc<dyn Any + Send + Sync>>,
    row_group_count: usize,
) -> Result<ParquetAccessPlan>

Return the initial ParquetAccessPlan

If the user has supplied one as an extension, use that otherwise return a plan that scans all row groups

Returns an error if an invalid ParquetAccessPlan is provided

Note: file_name is only used for error messages