Add subtensor.is_subnet_active method

[basfroman]

• adds is_subnet_active method
• adds e2e check
• adds unit test

[github-actions]

Bittensor SDK virtual environment sizes by Python version:

Python 3.9: 195 MB
Python 3.10: 195 MB
Python 3.11: 211 MB
Python 3.12: 205 MB
Python 3.13: 168 MB

[github-actions]

Bittensor SDK virtual environment sizes by Python version:

Python 3.9: 195 MB
Python 3.10: 195 MB
Python 3.11: 211 MB
Python 3.12: 205 MB
Python 3.13: 168 MB