[FLINK-13409][python] Supported java UDFs in python API.

[WeiZhong94]

What is the purpose of the change

This pull request aimed to support registering Java UDFs and using Java UDFs in Python Table API.

Brief change log

• Add register_java_function() and list_user_defined_functions() in TableEnvironment class to support registering Java UDFs.
• Add join_lateral() and left_outer_join_lateral() to support using UDTFs in python Table API.
• *Fix incorrect documentation of joinLateral interfaces. *

Verifying this change

This change is already covered by existing tests, such as test_correlate.py and test_table_environment_api.py.

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, Yarn/Mesos, 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? (python docs)

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Last check on commit d69fada (Tue Aug 06 15:50:15 UTC 2019)

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CI report:

• 69b8861 : SUCCESS Build
• d69fada : FAILURE Build

[dianfu]

@WeiZhong94 Thanks a lot for the PR. It would be better to also update the docs.

[dianfu]

The latest PR LGTM. +1 from my side.