Switch parquet library

[chelseajonesr]

• Switch parquet library used for checkpointing from https://github.com/segmentio/parquet-go to https://github.com/xitongsys/parquet-go
• (The previous library wrote nested optional strings in such a way that Spark 3.2 could not read the resulting parquet file)
• Update all action struct members to be pointers so that the schema members will be optional, to match the schema produced by Rust and Spark
• Update action parquet tags to match new library
• When reading checkpoints, parse actions using reflect
• Remove parsed fields from checkpoints for now
• Set minimum writer version to 1