feat(client): add count_by_dataset function to level0 and level1 clients

- Added count_by_dataset method to level0.py to query data counts grouped by dataset
- Added count_by_dataset method to level1.py to query data counts grouped by dataset
- Updated test_level0.py to include test case for count_by_dataset
- Updated test_level1.py to include test case for count_by_dataset
- Commented out multiple test methods in test_level0.py and test_level1.py