ARROW-15119: [C++] allow reading directories as files in generic tests

[coryan]

This is motivated by GCS, where checking if an object is a directory marker would add too much overhead.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-15119

[pitrou]

LGTM, thank you very much

[ursabot]

Benchmark runs are scheduled for baseline = 97a8159 and contender = e7c2ead. e7c2ead is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Scheduled] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.22% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/127| e7c2ead8 ec2-t3-xlarge-us-east-2>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/115| e7c2ead8 ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/118| e7c2ead8 ursa-thinkcentre-m75q>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/126| 97a81595 ec2-t3-xlarge-us-east-2>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/114| 97a81595 ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/117| 97a81595 ursa-thinkcentre-m75q>
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java