[FLINK-36957][Datastream] Implement asyc state version of stream flatmap

[fredia]

What is the purpose of the change

This PR implements the asyc state version of stream flatmap.

Brief change log

• Introduce StreamFlatMapAsync
• Update the behavior of KeyedStream#flatmap

Verifying this change

This change can be verified as follows:

• KeyedStateInputFormatTest

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 / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 6249060 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[davidradl]

@fredia I am unsure what this change is for - please could you add the motivation of the fix in the Jira and the impact on the Datastream API. Does this require a docs change?

[fredia]

@fredia I am unsure what this change it - please could you add the motivation of the fix in the Jira and the impact on the Datastream API. Does this require a docs change?

@davidradl Sorry for the confusion, this pr provides the async state version of stream flatmap, I changed the title and description.

[davidradl]

Reviewed by Chi on 09/01/2025 Need a committer to review

Thanks @fredia it looks ok from a quick glance - but would like someone more expert to review this

[Zakelly]

Thanks for the update, overall looks good!