[Python][Dataset] Writing dataset from python iterator of record batches

At the moment, from python you can write a dataset with ds.write_dataset for example starting from a list of record batches.

But this currently needs to be an actual list (or gets converted to a list), so an iterator or generated gets fully consumed (potentially bringing the record batches in memory), before starting to write.

We should also be able to use the python iterator itself to back a RecordBatchIterator-like object, that can be consumed while writing the batches.

We already have a arrow::py::PyRecordBatchReader that might be useful here.