[Parquet][C++] Optimize RecordReader for flat optional #50628

Describe the enhancement requested

When reading flat optional values from a Parquet file, the definition levels correspond to Arrow validity bitmap.
We can speed up RecordReader for this common case by decoding the levels directly as the validity bitmap.

Open questions:

• Should int16_t* RecordReader::def_levels() return nullptr (break) or make an effort to still provide some levels (additional complexity or cost). The new faster breaking behavior could also be enabled my a ctor flag.

Component(s)

C++, Parquet