Add a method to return the number of skipped rows in a RowSelection

[progval]

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

RowSelection has a row_count method that returns the number of selected rows, but is missing a way to count the number of de-selected rows without iterating on the selectors

Describe the solution you'd like

Implement it as RowSelection::skipped_row_count

Describe alternatives you've considered

current state, which is that users have to reimplement it themselves

Additional context

Datafusion had to implement it here:

https://github.com/apache/datafusion/blob/f2159e6cae658a0a3f561ec2d15ea948213fd0f8/datafusion/core/src/datasource/physical_plan/parquet/page_filter.rs#L271-L277

Suggested by @alamb here: apache/datafusion#12545 (comment)