Parquet Preserve BitMask

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Currently the parquet reader reads out definition levels into [i16] and then constructs a bitmask from this. In the case of a max definition level of 1, the data is likely encoded already as bitpacked with a bitwidth of 1. It is possible to just use this encoded representation as is, without decoding it to [i16] and then re-encoding it as a bitmask.

FWIW parquet2 performs this optimisation

Describe the solution you'd like

Reuse the already encoded bitmask directly