SDK: make the dataset cache directory customizable

[SpecLad]

Motivation and context

This is useful for people whose home directory is too small/not fast enough. It also lets us make the tests less hacky.

How has this been tested?

Unit tests. I also ran my old demos to make sure the default cache location remained the same.

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into CHANGELOG file
• [ ] I have updated the documentation accordingly
• I have added tests to cover my changes
• [ ] I have linked related issues (read github docs)
• [ ] I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.