[Parquet] Implement Variant type support in Parquet

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Parquet recently adopted the Variant type from Spark: https://github.com/apache/parquet-format/blob/master/VariantEncoding.md

Details on

• GH-455: Add Variant specification docs parquet-format#456

Describe the solution you'd like
I would like to implement variant support in parquet-rs

Describe alternatives you've considered

Additional context
I am not sure if any other parquet implementations have implemented this yet / if there are example parquet files. I will attempt to find out

[CurtHagenlocher]

There's an implementation in Spark (try here for starters) but when I last looked ~two months ago there was no binary test data; only some round trips via JSON.