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GH-34262: [C++][ORC] Support union type (#34416)

### Rationale for this change

The ORC adapter does not support union type yet.

### What changes are included in this PR?

Support union type to both ORC reader and writer.

### Are these changes tested?

To be added.

### Are there any user-facing changes?

No.
* Closes: #34262

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>