Function is_legacy_json2_extension_type 

pub fn is_legacy_json2_extension_type<T: AsRef<Field>>(field: T) -> bool

Checks whether this field uses the JSON2 extension layout from before type hints.

That layout used the same greptime.json extension name and json_structure_settings metadata as legacy JSONB. Its structured Arrow data type is therefore required to distinguish JSON2 from Binary JSONB.