[FLINK-35358][clients] Reintroduce recursive JAR listing in classpath load from "usrlib"

[ferenc-csaky]

What is the purpose of the change

With https://github.com/apache/flink/commit/e63aa12252843d0098a56f3091b28d48aff5b5af in Flink 1.19 an unintentional change were introduced, that now the DefaultPackagedProgramRetriever only loads JARs from the root level of the usrlib directory.

This change reverts that behavior to search directories in a recursive manner, and adds a test case to cover such scenario.

Brief change log

• Flies#list -> Files#walk in DefaultPackagedProgramRetriever.
• Add test case in DefaultPackagedProgramRetrieverITCase.

Verifying this change

Added relevant IT case.

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: yes
• The S3 file system connector: no

[flinkbot]

CI report:

• 4cd4046d28fecc8e65408c87fd17ac4929b42138 Azure: SUCCESS

Bot commands

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

[rkthtrifork]

@mbalassi @ferenc-csaky can we merge this and https://github.com/apache/flink/pull/24792 ?

[ferenc-csaky]

@mbalassi @ferenc-csaky can we merge this and #24792 ?

Yes, they should be merged.