[arrow-json] deserialize Variant fields

[debugmiller]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I would like to use arrow_json::Reader to deserialize json data into Variant.

Describe the solution you'd like
Currently arrow_json::Reader does not seem to support Variant Fields supplied in the Schema which makes it challenging to deserialize json. While the existing json_to_variant function only works to convert an entire string into a Variant, my use case is to convert into a Schema containing some standard DataTypes and some Variants.

Describe alternatives you've considered

Additional context