[FLINK-17345][python][table] Support register and get Python UDF in Catalog.

[WeiZhong94]

What is the purpose of the change

This pull request supports register and get Python UDF in catalog via sql ddl.

Brief change log

• Support register python catalog UDF via SQL DDL.
• Support create, get and alter python catalog UDF in HiveCatalog.

Verifying this change

This change is already covered by existing tests, such as PythonFunctionFactoryTest, HiveCatalogTest, GenericInMemoryCatalogTest.

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? (docs)

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Automated Checks

Last check on commit ecd6631 (Thu Apr 23 12:20:45 UTC 2020)

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CI report:

• 3579637 UNKNOWN
• 78e23cb Travis: SUCCESS Azure: SUCCESS

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[dianfu]

Could you explain the aim of this change? It seems to me there is no difference between the changed logic and the original logic.

[WeiZhong94]

The initial reason of this change is provide an approach in FunctionCatalog to register Python CatalogFunction. But it seems not necessary now because the upper layer accesses the Catalog object directly.

[dianfu]

What's the aim of this change?

[WeiZhong94]

Currently the Python UDF is always generic and can not be loaded via Class.forName. So just return true if the CatalogFunction is a Python UDF.

[WeiZhong94]

@dianfu Thanks for your review! I have addressed you comments in the latest commit.

[dianfu]

@WeiZhong94 Thanks a lot for the update. Will merge once the travis/azure turns green.