Fix missing / incomplete Parquet & Intake metadata about pudl-catalog

[zaneselvans]

Hey @katie-lamb I went ahead and assigned you to this one. I think it's a mix of problems stemming from missing pandas datatype metadata, and the way that Intake is interpreting & displaying the data it finds in the Parquet files.

The issues that I think are related to pandas metadata / writing Parquet files are:

• the string IDs unitid and unit_id_epa showing up as object columns.
• the fact that all the numeric types are standard Numpy types, rather than a mix of nullable and non-nullable pandas types based on what the original data types were. Like facility_id is allowed to be null, so it should be Int32 and not int32, while plant_id_eia can't be null, so it should be int32