[FLINK-37045][Runtime] Expose state creation APIs for state v2 to datastream(v1)

[Zakelly]

What is the purpose of the change

Currently, the state v2 is limited for internal use. This PR exposes those state APIs for DSv1 as the FLIP-488 discussed.

Brief change log

Please check the each commit.

• Move state descriptors to flink-core (move classes by IDE) -- Leave corresponding placeholder of state descriptor to make existing PRs work well. -- will remove after several days.
• Merge two versions of KeyedStateStore -- Thus many functions need not to change the signature while enabling state v2.
• Merge two versions of OperatorStateStore

Verifying this change

This change is already covered by existing tests.

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? yes
• If yes, how is the feature documented? no -- will add an intro doc of state v2 in later PR.

[flinkbot]

CI report:

• 96a9957 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

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