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

[ferenc-csaky]

What is the purpose of the change

With e63aa12 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:

• 4cd4046 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[mbalassi]

LGTM, will merge tomorrow if no objections until then.

[rkthtrifork]

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

[ferenc-csaky]

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

Yes, they should be merged.