[C++] Support float16 in writing/reading parquet

Half-float values are not supported in Parquet. Here is a previous issue that talks about that: https://issues.apache.org/jira/browse/PARQUET-1647

So, this will not work:

import pyarrow as pa
import pyarrow.parquet as pq
import numpy as np
arr = pa.array(np.float16([0.1, 2.2, 3]))
table = pa.table({'a': arr})
pq.write_table(table, "test_halffloat.parquet") 

This is a proposal to store float16 values in Parquet as FixedSizeBinary, and then restore them to float16 when reading them back in.