Enable dictionary encoding by default for enum ParquetFields

[clairemcginty]

Per the documentation:

The dictionary encoding builds a dictionary of values encountered in a given column. The dictionary will be stored in a dictionary page per column chunk. The values are stored as integers using the RLE/Bit-Packing Hybrid encoding. If the dictionary grows too big, whether in size or number of distinct values, the encoding will fall back to the plain encoding. The dictionary page is written first, before the data pages of the column chunk.

This seems like a good fit for Parquet enum types, which are written as strings under the hood. ParquetWriter.Builder has a .withDictionaryEncoding(String columnPath, boolean enableDictionary) that we could invoke for all enum ParquetFields.