Beta 3 Smart Port sensor demo implementation

Implements FrSky SmartPort protocoll (partially)

• STM32G031F8 and STM32G431CBU targets supported. Both microcontrollers support USART PIN inversion required for SmartPort bus (STM32F103 would not support this)
• PlatformIO plus Arduino based.
• provides the A3 Voltage sensor. Voltage values are taken from a loop counter not ADC yet.
• provides a 8s cell monitor sensor. The cell values are not measured just constants compiled in.
• provides GPS Lon, Lat, date, time packets (also dummy values - as of now)
• Serial DBG output displays the generated SmartPort packets (before the application of the "forbidden byte" stuffing modification)
• displays also valid SmartPort data packages from other sensors attacked too the SmartPort bus

Next steps: Real sensor implementation, GPS data read in, analog voltage, ...