arrow

PARQUET-1160: [C++] Implement BYTE_ARRAY-backed Decimal reads

This change enables support for DECIMALs backed by BYTE_ARRAYs on disk. It does this by creating a TransferFunctor routine that transforms a ByteArrayType to an ::arrow::Decimal128Type.

The routine does this by:
1. Creating an arrow::BinaryArray from the RecordReader's builder
2. Allocating a buffer for the arrow::Decimal128Array
3. Converting the big-endian bytes in each BinaryArray entry to two integers
   representing the high and low bits of each decimal value.

Author: Ted Haining <thaining@xcalar.com>

Closes #2646 from thaining/parquet-1160-byte-array-decimals and squashes the following commits:

0bad382e <Ted Haining> Updated parquet-testing to SHA that includes necessary test files.
30f3a278 <Ted Haining> This change enables support for DECIMALs backed by BYTE_ARRAYs on disk. It does this by creating a TransferFunctor routine that transforms a ByteArrayType to an ::arrow::Decimal128Type.