arrow

ARROW-14930: [C++] Make S3 directory detection more robust

When processing the results of a ListObjectsV2 request to determine whether something exists as a "directory", take the number of common prefixes into account in addition to the number of keys.

Also avoid issuing a second HEAD request on directories.

Tested on Minio, AWS and Zenko CloudServer.

Closes #11977 from pitrou/ARROW-14930-s3-dir-detection

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