Support non-struct root data type

[Jefffrey]

ORC spec: https://orc.apache.org/docs/types.html

Hive always uses a struct with a field for each of the top-level columns as the root object type, but that is not required

See datafusion-contrib/datafusion-orc#45 discussion

[Jefffrey]

Not even PyArrow supports this btw

So even lower priority