Add low-level String->Variant conversion support

[harshmotw-db]

Please describe why this is necessary.

Follow up for #1015

Similar to the parse_json expression in Apache Spark. This should be extended in the default engine to provide an arrow interface to translate a batch of strings into variants.
This would make implementing variant easier for engines interacting with kernel as they could use these utility functions.

Describe the functionality you are proposing.

No response

Additional context

No response