Add an option to disallow loading of dataset files outside dataset dir

[zhiltsov-max]

Can be useful for server-side usage of Datumaro to prevent security issues. Currently, Datasets that include image paths (and other paths), can use paths to files outside the dataset directory. It can lead to reading of the unwanted files, if the dataset is specially-crafted.

Possible ways to implement:

• Using a subprocess with chroot
• Wrapping open