Parquet: Add union method to RowSelection

[sdd]

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

RowSelection::intersection already exists for computing the intersection of two RowSelections. This is useful when you have a filter predicate that filters on two columns that have been AND'ed together and need the intersection of the RowSelection of each side of the AND.

Correspondingly, if you have a filter that is A OR B rather than A AND B, it is desirable to derive the union of the row selections of A and B.

Therefore I propose that RowSelection::union be added as a feature, to complement RowSelection::intersection.

Describe the solution you'd like

#3003 introduced RowSelection::intersection, for which #3047 was submitted and merged.

I will shortly be submitting a similar PR for RowSelection::union.

Describe alternatives you've considered

None

Additional context

I have a PR open on iceberg-rust, where I am using RowSelection::intersection but needed to implement a RowSelection union function myself. I think it makes more sense for it to live in here alongside intersection where others can make use of it too.

[alamb]

label_issue.py automatically added labels {'parquet'} from #6308