perf: cache `ArrayRef` in `RowGroupIndexReader` when row group doesn't change

[friendlymatthew]

• Related [parquet] Add row group index virtual column #9117

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

From @alamb

The other thing would be to pre-calculate the arrays (where the row group values don't change) and return the same ArrayRef until the RowGroup actually changes

[friendlymatthew]

I'll also plan on getting this over the line once #9117 merges