[C++][Parquet] Api inconsistency for bpacking32/bpacking64

[emkornfield]

Describe the enhancement requested

unpack32 takes int32* but unpack64 takes int8_t*. I think the unpack64 is correct since we can't rely on alignment and under the hood unpack32 does SafeLoad to avoid ubsan already. We should probably clean this up to make this clearer.

Component(s)

C++, Parquet