arrow

ARROW-16000: [C++][Python] Dataset: Alternative implementation for adding transcoding function option to CSV scanner (#13820)

This is an alternative version of https://github.com/apache/arrow/pull/13709, to compare what the best approach is.

Instead of extending the C++ ReadOptions struct with an `encoding` field, this implementations adds a python version of the ReadOptions object to both `CsvFileFormat` and `CsvFragmentScanOptions`. The reason it is needed in both places, is to prevent these kinds of inconsistencies:
```
>>> import pyarrow.dataset as ds
>>> import pyarrow.csv as csv
>>> ro =csv.ReadOptions(encoding='iso8859')
>>> fo = ds.CsvFileFormat(read_options=ro)
>>> fo.default_fragment_scan_options.read_options.encoding
'utf8'
```

Authored-by: Joost Hoozemans <joosthooz@msn.com>
Signed-off-by: David Li <li.davidm96@gmail.com>