Add KeypointsAnnotation and KeypointsPrediction support

Add support for keypoints annotations. Keypoints are defined as a collection of points, bundled with a skeleton that describes how they would physically connect when displayed on an image. This geometry is defined as:

• keypoints: a list of {x, y, visible} objects that indicate an individual vertex
• names: a list of names that correspond to the keypoints in this label
• skeleton: a list of line segments that indicate the connections between the keypoints