RecordBatchReader.from_batches: accept Iterator in addition to Sequence

[kevinjacobs-delfi]

RecordBatchReader.from_batches currently requires a Sequence (maps to Vec<PyRecordBatch> in PyO3), but the underlying RecordBatchIterator::new only needs an iterator. This makes it impossible to stream lazily from a Python generator into write_parquet without materializing all batches in memory first.

Would it be possible to add a constructor that accepts a Python iterator/generator? Something like:

def gen():
    for chunk in chunks:
        yield RecordBatch(...)

reader = RecordBatchReader.from_batches(schema, gen())
write_parquet(reader, path, ...)

Happy to submit a PR if you're open to it.