[FLINK-37924][table]Introduce Built-in Function to Access field or element in the Variant

[JinkunLiu]

What is the purpose of the change

Introduce a built-in function to access a field or element in a Variant. This is part of FLIP-521.

SQL Function	Table Function	Description
variant ‘[’ INT ‘]’	VARIANT.at(INT)	If the VARIANT is an ARRAY value, returns a VARIANT whose value is the element at the specified index. Otherwise, this operation returns NULL
variant ‘[’ STRING ‘]’	VARIANT.at(STRING)	If the VARIANT is a MAP value that has an element with this key, a VARIANT holding the associated value is returned. Otherwise, NULL is returned.

Brief change log

• Support accessing VARIANT elements using [INT] or VARIANT.at(INT) when the VARIANT contains an ARRAY value in SQL or table expressions.
• Support accessing VARIANT elements using ['STRING'] or VARIANT.at('STRING') when the VARIANT contains a MAP value in SQL or table expressions.

Verifying this change

This change added tests and can be verified as follows:

• Added six new test at org.apache.flink.table.planner.plan.nodes.exec.stream.VariantSemanticTest

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)

[flinkbot]

CI report:

• 26d8f28 UNKNOWN
• 80d4638 Azure: FAILURE

Bot commands

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

[JinkunLiu]

Hello @Sxnan Could you take a look at this PR when you have time? Thanks

[yunfengzhou-hub]

Thanks for the PR @JinkunLiu. Left some comments as below.

[JinkunLiu]

Hi @yunfengzhou-hub, thanks for the review! I have updated the code according to your suggestions. Could you please take another look at this PR when you have time? Thank you very much!

[yunfengzhou-hub]

Thanks for the update. Generally LGTM. Some minor comments are as below.

[JinkunLiu]

Hi @davidradl , thanks for the review! Could you take an another look. Thanks

[xuyangzhong]

Thanks for driving this. This pr LGTM overall. I just left some minor comments.

[xuyangzhong]

Thanks for updating, just one more minor comment.

[xuyangzhong]

+1 👍

[yunfengzhou-hub]

+1