arrow

GH-43221: [C++][Parquet] Refactor parquet::encryption::AesEncryptor to use unique_ptr (#43222)



### Rationale for this change

See https://github.com/apache/arrow/issues/43221

### What changes are included in this PR?

Change raw-pointer to unique_ptr

### Are these changes tested?

Covered by existing

### Are there any user-facing changes?

Maybe change user interface

* GitHub Issue: #43221

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>