[FLINK-38615][python] Optimize AsyncFunction to remove ResultFuture

[dianfu]

What is the purpose of the change

• This pull request optimizes AsyncFunction interface to remove ResultFuture. *

Brief change log

• *Removes ResultFuture from the signature of AsyncFunction *

Verifying this change

This change is already covered by existing tests in test_async_function.py*.

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)

[flinkbot]

CI report:

• 13de53d Azure: FAILURE
• fdedefc Azure: PENDING

Bot commands

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[davidradl]

I think we should deprecate the old API and introduce this, in this way exiting applications will not break and the older API version can be managed out. This is less a consideration for master - but is a consideration for the backports.

[dianfu]

@davidradl Good point! This is introduced in Flink 2.2 and so there is no backward compatibility issues (I'd like to make sure this is merged into Flink 2.2).

[bgeng777]

The change is great as it is more pythonis. My only concern is that the exception propagation is somehow different.

[bgeng777]

As we get rid of 'result_future' in the AsyncFunction, we may change the error description here as well when users return non-iterable result

[bgeng777]

I modify the test a little to return None and I find that the test can run without throwing The 'result_future' of AsyncFunction should be completed with ... exception.
Is that expected?

[bgeng777]

Thanks for the great work, LGTM!