ARROW-8201: [Python] Add FileFragment.open() method

[milesgranger]

Fixes ARROW-8201

[github-actions]

https://issues.apache.org/jira/browse/ARROW-8201

[pitrou]

Also cc @jorisvandenbossche

[jorisvandenbossche]

(renamed the PR since it is just about adding an open() now)

[ursabot]

Benchmark runs are scheduled for baseline = cd1c01c and contender = ece5b65. ece5b65 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️1.11% ⬆️0.0%] test-mac-arm
[Failed ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.04% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] ece5b654 ec2-t3-xlarge-us-east-2
[Failed] ece5b654 test-mac-arm
[Failed] ece5b654 ursa-i9-9960x
[Finished] ece5b654 ursa-thinkcentre-m75q
[Finished] cd1c01cf ec2-t3-xlarge-us-east-2
[Failed] cd1c01cf test-mac-arm
[Failed] cd1c01cf ursa-i9-9960x
[Finished] cd1c01cf 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