[C++][Parquet] Make DELTA_BYTE_ARRAY value reconstruction storage-agnostic #50822

Describe the enhancement requested

Follow-up to #50335, which added FLBADecoder::Decode(uint8_t*, int) - a dense decode
path that writes FIXED_LEN_BYTE_ARRAY values contiguously into a caller-provided buffer.

DeltaByteArrayFLBADecoder::Decode(uint8_t*, int) currently reaches that dense output
through DeltaByteArrayDecoderImpl::GetInternal, which only supports ByteArray output.
So it decodes into a temporary std::vector<ByteArray>, then copies each value out and
discards the vector.

DeltaByteArrayDecoderImpl::DecodeArrow allocates a similar intermediate ByteArray
buffer, so it may benefit from the same refactor.

Component(s)

C++