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ARROW-453: [C++] Filesystem implementation for Amazon S3

Unit testing is done using the Minio standalone S3 server.

Currently missing from this PR:
- fix the bundled building (currently it links against the wrong version of `libcrypto`, ending up loading two different OpenSSL versions at runtime)

Closes #5167 from pitrou/ARROW-453-s3fs and squashes the following commits:

de1795ac6 <Antoine Pitrou> ARROW-453:  Filesystem implementation for S3

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