[SQL] JSON support

[mihaibudiu]

JSON will be supported in SQL through a VARIANT data type inspired by the design in Snowflake: https://docs.snowflake.com/en/sql-reference/data-types-semistructured

• [x] Add VARIANT data type in Calcite https://issues.apache.org/jira/browse/CALCITE-4918, https://github.com/apache/calcite/pull/3947
• [x] Get the implementation reviewed
• [ ] Get the implementation merged
• [x] Support VARIANT in the SQL compiler: #2421
• [x] Support VARIANT in the runtime library
• [x] Implement JSON parsing functions that return VARIANT values
• [ ] Implement other JSON functions that operate on VARIANT values that are expected to represent JSON documents

[mihaibudiu]

Calcite PR with variant support submitted for review: https://github.com/apache/calcite/pull/3947 We can start already building on our side. The most complicated will be runtime support in Rust.

[mihaibudiu]

See #2453