Add interface for box prompt in SAM 2 video predictor (#174)

This PR adds an example to provide box prompt in SAM 2 as inputs to the `add_new_points_or_box` API (renamed from`add_new_points`, which is kept for backward compatibility). If `box` is provided, we add it as the first two points with labels 2 and 3, along with the user-provided points (consistent with how SAM 2 is trained).

The video predictor notebook `notebooks/video_predictor_example.ipynb` is updated to include segmenting from box prompt as an example.