Implement labels argument for create_dataset (and downstream functions)

[JosuaRieder]

Implement support for providing the labels with a json file for the "Folder" dataset. Support for other file formats could also be added easily.

I train on pseudo-labels and having to realize the pseudo-labeling using the file system is very slow.

It was not entirely clear to me at which level I should load the actual file, i.e. when to convert the Union[Dict, str] to Dict. I decided on create_reader but it could theoretically also be done one level later, i.e. in find_images_and_targets.