[FLINK-21649][state/heap] Allow extension of CopyOnWriteState classes

[rkhachatryan]

What is the purpose of the change

Allow extension of CopyOnWriteState classes
so that incremental versions can be added.

Please see https://github.com/rkhachatryan/flink/tree/flip-151-full
for the context of how this change is used.

Verifying this change

This change is a trivial rework 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/Mesos, 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? no

[flinkbot]

CI report:

• f0e9c27 Azure: FAILURE

Bot commands

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

[rkhachatryan]

CI failures unrelated (FLINK-26674), tests in private branch passed.