Add convenience function for limit/offset

[changhiskhan]

Right now to take advantage of Lance performance, we need to explicitly call lance.scanner(..., limit=20, offset=10). It would be nice to be able to do something like lance.dataset(uri).limit(n=20, offset=10).

Note that pyarrow's Dataset already has a head method and under the hood it's creating a scanner. BUT, it does not support Lance's scanner and limit/offset options. In addition, pyarrow's Dataset and Scanner are cython cdef classes so cannot be monkey patched.

Options:

1. Create Lance subclasses for FileSystemDatasetFactory and FileSystemDataset (both in C++ and cython) which returns the right scanner and supports more ScanOptions. We would then monkey-patch pyarrow.dataset._filesystem_dataset to use this new factory and dataset.
2. Create cython-only LanceFileSystemDataset with a factory method to create an instance from a FileSystemDataset. Then in lance.dataset, call this factory method so lance.dataset returns a Dataset that supports Dataset.head(n=20, offset=10), which calls lance.scanner under the hood to support lance-specific ScanOptions.