GH-34063: [C++] Avoid waste in GcsFileSystem::ReadAt()

[coryan]

With this change GcsFileSystem::ReadAt() will only request exactly the bytes it needs.

Fixes #34063.

Are these changes tested?

There are existing tests for ReadAt() this is just a performance change.

Are there any user-facing changes?

No.

• Closes: [C++] Stop downloads ASAP in GcsFileSystem #34063

[github-actions]

• Closes: [C++] Stop downloads ASAP in GcsFileSystem #34063

[github-actions]

⚠️ GitHub issue #34063 has been automatically assigned in GitHub to PR creator.

[coryan]

I am attributing the Python / AMD64 Conda Python 3.9 Sphinx & Numpydoc failure to the problems fixed in #34038.

[lidavidm]

Thanks!

[ursabot]

Benchmark runs are scheduled for baseline = afbee08 and contender = 6b65c84. 6b65c84 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Failed ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.43% ⬆️0.0%] test-mac-arm
[Failed ⬇️0.26% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.13% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Failed] 6b65c84a ec2-t3-xlarge-us-east-2
[Failed] 6b65c84a test-mac-arm
[Failed] 6b65c84a ursa-i9-9960x
[Finished] 6b65c84a ursa-thinkcentre-m75q
[Failed] afbee088 ec2-t3-xlarge-us-east-2
[Failed] afbee088 test-mac-arm
[Failed] afbee088 ursa-i9-9960x
[Finished] afbee088 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