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GH-38794: [C++][S3] Handle conventional content-type for directories (#40147)

### Rationale for this change

Some AWS-related tools write and expect the content-type "application/x-directory" for directory-like entries.

This PR does two things:
1) set the object's content-type to "application/x-directory" when the user explicitly creates a directory
2) when a 0-sized object with content-type starting with "application/x-directory" is encountered, consider it a directory

### Are these changes tested?

Unfortunately, this cannot be tested with MinIO, as it seems to ignore the content-type set on directories (as opposed to regular files).

### Are there any user-facing changes?

Hopefully better compatibility with existing S3 filesystem hierarchies.

* Closes: #38794
* GitHub Issue: #38794

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>