[FLINK-37932] Add UserAgent in GCS requests

[shrutisinghania]

What is the purpose of the change

This pull request adds a User-Agent header to all Google Cloud Storage (GCS) requests made by the Flink GCS file system. This allows for better tracking and identification of Flink-originated traffic on the GCS side.

Brief change log

• In GSFileSystemFactory.java, a User-Agent header is added to the StorageOptions.Builder for the GCS client

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): No
• The public API, i.e., is any changed class annotated with @Public(Evolving): No
• The serializers: No
• The runtime per-record code paths (performance sensitive): No
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: No
• The S3 file system connector: No

Documentation

• Does this pull request introduce a new feature? No
• If yes, how is the feature documented? Not applicable

[flinkbot]

CI report:

• 6b029f9 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[shrutisinghania]

Hi @twalthr, this is my first commit in Flink. I saw you have been actively contributing to this repository and hence I was wondering if you could take a look at this PR and share your thoughts!