Add `as_iterable_dataset()` method to DatasetBuilder for streaming from cached Arrow files
This PR implements `builder.as_iterable_dataset(split=...)` as discussed in #5481.

It allows users to load an `IterableDataset` directly from cached Arrow files (using ArrowReader and ArrowExamplesIterable), without loading the full dataset into memory.

This is useful for large-scale training scenarios where memory is constrained. A test has also been added in `test_builder.py`.

Related to: #5481
