[FLINK-29028][python][doc] Add the missing cache api in Python DataStream API

[HuangXingBo]

What is the purpose of the change

This pull request will add the missing cache api in Python DataStream API

Brief change log

• Add the missing cache api in Python DataStream API

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:

• 7bc0756 Azure: FAILURE

Bot commands

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

[dianfu]

I guess we may need to return CachedDataStream which has some specific API, e.g. invalidate.

[Sxnan]

On top of that, we also introduce SideOutputDataStream which supports the cache method. I think we may need to introduce SideOutputDataStream in PyFlink and let get_side_output return SideOutputDataStream.

[HuangXingBo]

@Sxnan I think we can reuse the cache method of DataStream, no need to introduce a new class SideOutputDataStream

[dianfu]

Need also add close method in StreamExecutionEnvironment.

[HuangXingBo]

@dianfu @Sxnan I have resolved the comments in the latest commit.

[Sxnan]

@HuangXingBo Thanks for your update! No more comments from my side.