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GH-38663: [C++] Add support for service-specific endpoint for S3 using `AWS_ENDPOINT_URL_S3` (#39160)

### Rationale for this change

See #38663

### What changes are included in this PR?

set variable `endpoint_override` according the environment variable, prefer service-specific endpoint url over global endpoint url.

### Are these changes tested?

unittest

### Are there any user-facing changes?

No

* Closes: #38663

Lead-authored-by: messense <messense@icloud.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>