[C++][Parquet] Add a dense decoder for FIXED_LEN_BYTE_ARRAY values #50333

Describe the enhancement requested

FLBADecoder currently only exposes Decode(FixedLenByteArray*, int), which writes one FixedLenByteArray pointer per value.
Callers that only need the raw values must first materialize these per-value pointers, then copy the underlying bytes out through them.

For performance reasons, it would be useful to have a decode overload that writes the decoded FIXED_LEN_BYTE_ARRAY values directly into a caller-owned, densely packed buffer.

Component(s)

Parquet