Added firmware sensor

The firmware sensor has been added because it was discovered that an earlier firmware version was causing issues with this integration (and the EMAmanager app). Hopefully, any issues can then be attributed more quickly to a firmware update might this happen in the future.

New dedicated sensor that tracks the ECU firmware version

• Displays current firmware version with diagnostic entity category
• Supports state restoration between Home Assistant restarts
• Auto-updates when firmware changes are detected during polling