[FR] Include additional attributes in Cityscapes Dataset

The Cityscapes dataset does not currently load all available attributes, such as visible bounding box and visibility ratio of the person annotations, but some users may want these additional attributes to be loaded, similar to how the COCO dataset loads area and iscrowd attributes.

Here's where the person annotations are parsed in the implementation: https://github.com/voxel51/fiftyone/blob/bf03dde6e4a0becabbce9e8fbe6fe82aa0a0f9be/fiftyone/utils/cityscapes.py#L416

It should be easy to add the extra attributes as additional Detection attributes in there.

Here's where the cityscapes dataset is implemented in the zoo: https://github.com/voxel51/fiftyone/blob/bf03dde6e4a0becabbce9e8fbe6fe82aa0a0f9be/fiftyone/zoo/datasets/base.py#L671

It might make sense to add an optional argument to the CityscapesDataset class to control whether/which the additional attributes are loaded. Not sure.