[FLINK-37923][sql] Introduce VARIANT type and PARSE_JSON to Flink SQL

[Sxnan]

What is the purpose of the change

This pull request introduces the Variant data structure to represent semi-structured data, a new SQL type variant, and a builtin method to parse json string to the variant type.

Brief change log

• Patch calcite to support variant type
• Introduce Variant and BinaryVariant
• Introduce variant type and PARSE_JSON to Flink SQL

Verifying this change

This change added tests and can be verified.

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): yes
• 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? yes
• If yes, how is the feature documented? JavaDocs

[flinkbot]

CI report:

• 5c1c877 Azure: SUCCESS

Bot commands

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

[twalthr]

Awesome work @Sxnan! I left a couple of comments that should improve consistency when integrating this type into the existing type system.

[twalthr]

If we want to format timestamps according to JSON we should actually use T in-between and Z at the end for TIMESTAMP_LTZ.
https://stackoverflow.com/questions/10286204/what-is-the-right-json-date-format

[twalthr]

See also https://nightlies.apache.org/flink/flink-docs-master/docs/connectors/table/formats/json/#json-timestamp-format-standard.
Or is the the intention to use SQL format, which is also fine for me. I just wanted to raise awareness here.

[Sxnan]

Good point! I think we can follow the specification of the Variant type here, which uses T in-between for TIMESTAMP but no Z at the end for TIMESTAMP_LTZ.

[Sxnan]

@twalthr Thanks for the detailed review! I updated the PR accordingly, and all the comments should be addressed. Please take another look.

[twalthr]

Thanks @Sxnan. I added it for my list for tomorrow. I'm sure it can still make it before the feature freeze.