[arrow-json] deserialize Variant fields

**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](https://arrow.apache.org/rust/parquet_variant_compute/from_json/fn.json_to_variant.html) 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**

Start with the arrow_json::Reader entry point and compare its schema handling with the json_to_variant function linked in the issue. Trace how fields are deserialized when a schema mixes standard DataTypes and Variant fields. Done means JSON data can be read into that mixed schema, including the Variant values, with coverage for the behavior.

Written by the indexing model from the issue text.