Refactor imports and enhance gNMI collector functionality

- Updated import paths for _Collector in INTCollector.py.
- Added missing scapy dependency in requirements.in.
- Changed return types to Optional in HelperMethods.py.
- Improved disconnect handling in GnmiOpenConfigCollector.py.
- Added TOTAL_POWER KPI in KPI.py.
- Enhanced PathMapper.py for wavelength router support.
- Refined SubscriptionNew.py with graceful stop and response parsing.
- Updated test cases in messages.py and test_unit_GnmiOpenConfigCollector.py for better logging and parameter handling.