Consider Adding "on" and "how" support to read_parquet

[wilsonbb]

Feature request
read_parquet supports specifying additional parquet files to pack in with an explicit add_nested call. Given the work in #120, we may want to consider adding the ability to specify "on" and "how" in the dictionary used in read_parquet's to_pack parameter. We could consider adding additional dictionary-based parameters similar to pack_columns (in this case pack_on and pack_how).

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