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GH-34363: [C++] Use equal size parts in S3 upload for R2 compatibility (#35808)

### Rationale for this change

S3 and Minio allow varied part sizes, but R2 doesn't yet. So for now we'll change to only write 

### What changes are included in this PR?

Alters the `DoWrite` implementation to used fixed size parts.

### Are these changes tested?

* [x] Tested manually against R2
* [x] Tested manually against S3

### Are there any user-facing changes?

* Closes: #34363

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>