Support decoding Union column with up to including 256 variants

[Jefffrey]

According to https://orc.apache.org/specification/ORCv1/

Currently ORC union types are limited to 256 variants, which matches the Hive type model.

However in Arrow, UnionArrays are limited to 127 variants: https://arrow.apache.org/docs/format/Columnar.html#union-layout

A union with more than 127 possible types can be modeled as a union of unions.

To support this, would need to do as above and decode into union of union

See initial Union support here: datafusion-contrib/datafusion-orc@ee69b91

[suxiaogang223]

I'd like to pick up this :)