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ARROW-6362: [C++] Allow customizing S3 credentials provider

Also provide API facilities to use the default provider chain
(looks up environment variables and config files), or a hard-coded
access / secret key pair.

Closes #5368 from pitrou/ARROW-6362-s3-credentials and squashes the following commits:

e104c73ba <Antoine Pitrou> Flip S3's narrative test S3GlobalOptions.log_level with `--verbose`.
05d8508c0 <Antoine Pitrou> ARROW-6362:  Allow customizing S3 credentials provider

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