GH-50148: [C++] Add Content-Encoding support to S3 filesystem metadata

[alytantawyy]

Rationale for this change

The S3 filesystem metadata handling supported headers such as Content-Type,
Content-Language, Cache-Control, and Expires, but omitted
Content-Encoding.

As a result, Content-Encoding was not propagated when writing S3 object
metadata, and it was also not returned when reading metadata back.

What changes are included in this PR?

• add Content-Encoding to S3 object metadata extraction
• add Content-Encoding to the S3 metadata setter whitelist
• add a dedicated setter for Content-Encoding
• extend the S3 metadata round-trip test to cover explicit and default
  Content-Encoding metadata

Are these changes tested?

Yes.

./cpp/build-s3-system/debug/arrow-s3fs-test --gtest_filter=TestS3FS.OpenOutputStreamMetadata

* GitHub Issue: #50148