[FLINK-33387][runtime] Introduce the abstraction and the interface about loading

[RocMarshal]

What is the purpose of the change

Introduce the abstraction and the interface about loading
Support tasks balancing at TaskExecutor level for Default Scheduler without public users-oriented interface for enabling the feature.

Brief change log

[FLINK-33388][runtime] Support tasks balancing at TaskExecutor level for Default Scheduler [FLINK-33387][runtime] Introduce the abstraction and the interface about loading

Verifying this change

Added the corresponding test cases about core logic.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented yet, will do it.)

[flinkbot]

CI report:

• 2a34f6596fd8295b185bbbab61b7879889f6b3df Azure: SUCCESS

Bot commands

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

[RocMarshal]

Hi @1996fanrui could you help take a look if you had the free time ? thx a lot~

[RocMarshal]

the new pr is in https://github.com/apache/flink/pull/25734