[FLINK-15974][python] Support to use the Python UDF directly in the Python Table API

[dianfu]

What is the purpose of the change

This pull request add support to use the Python UDF directly in the Python Table API. E.g. for Python UDF inc, users could use it directly in the Python Table API: tab.select(inc(tab.a)).insert_into("sink")

Brief change log

• Add call method in UserDefinedFunctionWrapper which will allow to call the user-defined function directly in the Python Table API program
• Improve expressions.call method to support TableFunction in the old planner.
• Fix FlinkTypeFactory which doesn't works well with the old Decimal TypeInformation in blink planner.
• Update the tests in test_udf.py, test_udtf.py, test_pandas_udf.py to use this feature.

Verifying this change

This change is already covered by the updated existing tests test_udf.py, test_pandas_udf.py, etc.*.

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/Mesos, 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? (Will add documentation in a separate PR)

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Automated Checks

Last check on commit 26abf1e (Fri Sep 04 05:34:06 UTC 2020)

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CI report:

• 6f65076 Azure: SUCCESS

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[WeiZhong94]

@dianfu Thanks for your PR! Look good overall, just some minor comments.

[WeiZhong94]

Can be replaced by pyflink.util.utils.load_java_class()?

[WeiZhong94]

add a type hint?

[dianfu]

@WeiZhong94 Thanks for the review. Updated the PR.