[FLINK-26044][table] Properly declare an internal UNNEST function

[twalthr]

What is the purpose of the change

This converts UNNEST into a registered internal function that will be specialized two times until a UDF is known.

Brief change log

Use SpecializedFunction to lazy instantiate a runtime implementation from an internal UDF name.

Verifying this change

This change is already covered by existing tests.

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? JavaDocs

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CI report:

• e0044ea Azure: FAILURE

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Last check on commit e0044ea (Wed Feb 09 13:10:17 UTC 2022)

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[slinkydeveloper]

Just a minor style comment

[slinkydeveloper]

Can you replace this method call here with the actual logic? e.g. here becomes arrayType.getElementType(). It's easier to read and helps understanding this method