Support decoding Union column into DenseUnion

[Jefffrey]

Initial Union support will decode to Sparse Union: datafusion-contrib/datafusion-orc@ee69b91

Can try give option to prefer decoding to DenseUnion: https://arrow.apache.org/docs/format/Columnar.html#dense-union

• Since ORC already encodes data in a dense manner (omitting nulls in the data streams) so might be more efficient? Unknown, just something to keep in mind