[FLINK-35356][State] State descriptor and implementation for async reducing state

[Zakelly]

What is the purpose of the change

This PR provides the definition of StateDescriptor and simple implementation for ReducingState in State V2.

Brief change log

• Definition and tests for ReducingState and ReducingStateDescriptor

Verifying this change

Added UT for each new introduced class.

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? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• f1f14a7cc3d9b6485d45e4d1b9884d58292c09c3 Azure: SUCCESS

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