[Variant] Make `VariantArray` iterable

[friendlymatthew]

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

Currently, there is no way to safely iterate over VariantArray. Only VariantArray::value(i) is available, which panics on null values.

Additional context

This feature is motivated by datafusion-variant, a higher-level library that integrates with Datafusion through a series of udfs

[friendlymatthew]

take

[alamb]

label_issue.py automatically added labels {'parquet-variant'} from #8613